The rDNA Transcription Machinery Is Assembled during Mitosis in Active NORs and Absent in ...

12 Pages·2002·5.52 MB·English

The rDNA Transcription Machinery Is Assembled during
Mitosis in Active NORs and Absent in Inactive NORs
Pascal Roussel,* Chantal Andr6,* Lucio Comai, I~ and DaniEle Hernandez-Verdun*
*Institut Jacques Monod, 75251 Paris Cedex ,50 France;*Laboratoire d'Immunologie Biologique, H6pital Henri Mondor, 01049
Cr6teil, France; §Howard Hughes Medical Institute, Department of Molecular and Cell Biology, University of California ta'
Berkeley, Berkeley, California ;4023-02749 and 1lDepartment of Molecular Microbiology, HMR-509, University of Southern
California School of Medicine, Los Angeles, California 33009
Abstract. In cycling cells, the rDNAs are expressed interphase cells as well as in mitotic cells. These results
from telophase to the end of G2 phase. The early re- clearly indicated that the RNA polymerase I complex
sumption of rDNA transcription at telophase raises the remained assembled during mitosis. In addition, RNA
D
question of the fate of the rDNA transcription machin- polymerase I and the transcription factors varied in the ow
n
ery during mitosis. At the beginning of mitosis, rDNA same proportions in the positive NORs, suggesting sto- loa
d
transcription is arrested, and the rDNAs are clustered ichiometric association of these components. The fact ed
in specific chromosomal sites, the nucleolar organizer that the rDNA transcription machinery is not equally from
h
regions (NOR). In human cells, we demonstrate that distributed among NORs most likely reflects the impli- ttp
the rDNA transcription machinery, as defined in vitro, cation of the different NORs during the subsequent in- ://rup
is colocalized in some NORs and absent from others terphase. Indeed, we demonstrate that only positive ress.o
whatever the mitotic phase: RNA polymerase I and the NORs exhibit transcription activity at telophase and rg
RNA polymerase I transcription factors, upstream that the level of transcription activity is related to the /jcb/a
binding factor and promoter selectivity factor (as veri- amount of rDNA transcription machinery present in rticle
fied for TATA-binding protein and TATA-binding the NOR. We propose that assembly of rDNA tran- -pd
f/1
protein-associated factor for RNA polymerase I 110), scription machinery preceding mitosis determines ex- 33
/2
were colocalized in the same NORs. The RNA poly- pression of the rDNAs at the beginning of the next cell /23
5
merase I complex was localized using two different an- cycle. Consequently, the association of rDNAs with the /12
6
tibodies recognizing the two largest subunits or only the rDNA transcription machinery defines the "active" 55
8
3
third largest subunit, respectively. These two antibodies NORs and the level of activity at the transition telo- /2
3
5
immunoprecipitated the RNA polymerase I complex in phase/interphase. .p
d
f b
y g
u
e
st o
n
1
I N eukaryotic cells during interphase, nucleoli are the (TIF) IB, is a complex composed of the TATA-binding 1 M
a
sites of rDNA transcription, rRNA processing, and protein (TBP) and three TBP-associated factors (TAF) (7, rch
the assembly of ribosomes (19, 50). In human cells, 8, ,11 39, 54). UBF and SL1 most likely represent the mini- 20
2
3
the rDNA transcription machinery, as defined in vitro, is mal set of factors needed in addition to RNA pol I to di-
composed of RNA polymerase I (RNA pol 1) 1 in associa- rect rDNA transcription in human cells. Indeed, in addi-
tion with the RNA pol I transcription factors upstream tion to UBF and SL1 (designated TIF-IB in mice), two
binding factor (UBF) and promoter selectivity factor transcription factors designated TIF-IA and TIF-IC were
(SL1) (2, ,3 24). The latter transcription factor or the cor- identified in mouse cells. TIF-IA is a growth-dependent
responding murine factor, transcription initiation factor transcription initiation factor. It interacts physically with
RNA pol I and converts it into a transcriptionally active
RNA pol I (41). TIF-IA was purified from exponentially
Address all correspondence to Dani~le Hernandez-Verdun, Institut growing mouse cells and identified in HeLa cells (42). TIF-
Jacques Monod, 2 place Jussieu, 75251 Paris Cedex 05, France. Tel. (33) 1 IC, implicated in both transcription initiation and elonga-
44 27 40 38. Fax: (33) 1 44 27 59 94. e-mail: [email protected].
tion of RNA pol I (44), has not yet been identified in hu-
1. Abbreviations used in this paper: BrUTP, bromo-uridine 5'-triphospate; man cells.
DAPI, 4',6-diamidino-2-phenylindole; NOR, nucleolus organizer region; However, even if the partners necessary to promote
RNA pol I, RNA polymerase I; RT, room temperature; SL1, promoter se- transcription of rDNAs have been recently well character-
lectivity factor; TAF, TBP-associated factor; TAF1, TBP-associated factor
ized in vitro, this characterization is not sufficient to pre-
for RNA polymerase I; TBP, TATA-binding protein; TIF, transcription
dict how the rDNA transcription machinery proceeds in
initiation factor; UBF, upstream binding factor.
© The Rockefeller University Press, 0021-95251961041235112 $2.00
The Journal of Cell Biology, Volume 133, Number 2, April 1996 235-246 235
vivo. In particular, it would be essential to know the fate of ditions to compare the rDNA machinery associated with
the rDNA transcription machinery when ribosomal tran- the different types of NORs. In this communication, we
scription is repressed. To address these questions, we took demonstrate that the rDNA transcription machinery, as
advantage of the physiological conditions provided by the defined in vitro, is colocalized in some NORs and absent
cell cycle during which there are alternate phases of ex- from others whatever the mitotic phase. The results show
pression and repression of the rDNAs. In cycling cells, the that RNA pol I and SL1 complexes are maintained during
rDNAs are expressed from telophase to the end of inter- mitosis. In addition, RNA pol I and the transcription fac-
phase and are repressed during mitosis (34). In prophase tors varied in the same proportions in the positive NORs,
while rRNA synthesis is down-regulated, nucleoli disap- suggesting stoichiometric association of these components
pear. The reformation of nucleoli occurs in telophase at in complexes.
specific chromosomal sites designated nucleolar organizer
regions (NOR) (25, 33) and requires rDNA transcription
Materials and Methods
(4, 5, 13). Thus, NORs are the sites where the rDNA tran-
scription machinery is activated in telophase and conse-
Purified Proteins, Antisera, and Antibodies
quently, the sites where the mitotic inactive state should
be investigated to understand the transition between mito- The mouse and yeast RNA pol I were provided respectively by .1 Grummt
sis and interphase. (German Cancer Research Center, Heidelburg, Germany) and A.
NORs are characterized by the presence of rDNA clus- Senenac and M. Riva (CEA, Saclay, France). The mouse anti-TBP mAb
(16E8) was obtained from R. Tjian and R. Weinzierl (University of Cali-
ters and of several nonhistone proteins. In humans, NORs
fornia, Berkeley, CA), and the rabbit polyclonal anti-TBP antibodies pre-
are located in the secondary constrictions of the 10 acro- pared against the full-length recombinant human protein were from Santa
centric chromosomes (chromosome pairs 13, 14, 15, 21, Cruz Biotechnology (Santa Cruz, CA). The mouse anti-bromodeoxyuri- D
and 22) (references ,1 ,21 20, 47). The 400 copies of rDNAs dine mAb was from Boehringer Mannheim France S.A. (Meylan, France). ow
n
found in humans are distributed in a nonuniform manner The polyclonal rabbit anti-TAF~ll0 (TBP-associated factor for RNA poly loa
I 110) antibodies, kindly provided by H. Beckmann (University of Cali- de
on these different chromosomes (26). In addition "active" d
and "inactive" mitotic NORs were defined by the pres- ftoorniima,m une Berkeley, serum withC A), specificity were described against previousUlBy F (A17) (54). was The described human (38). au- from
h
ence of a class of proteins designated Ag-NOR proteins. Two other autoimmune sera (A18 and Vll) were characterized in the ttp
These proteins were found associated only with the NORs present study. These autoimmune sera contained only autoantibodies ://ru
in which the rDNAs were to be efficiently expressed dur- characterized as IgGs and directed against nucleolar proteins. The anti- pre
ing interphase (30, 31). These observations indicated that b1o0%d ies SDS-polyacrywleraem ide affinity purified gels using and nucleolar electrotransferred. proteins electrophoresAendt ibodies were in ss.org
even during mitosis, in the absence of any transcription eluted by incubation in 100 mM glycine, pH 3, at room temperature (RT) /jcb
a(3s4,s ociated 51), there to NORs are differednecpese nding ion n thet heir nonhistone function during proteins in- foboofrd ies, 1 10 M min. Triwsit-hH Cl, The specificity eluted pH 8. fractfioro ns FITC- rabbit and were or Texas mouse neutralized red--conjugateIdg Gs anbdy adding for human secondary 10% IgG(sv,o l/vol) anti- re- /article-pd
terphase. Nonhistone proteins associated with NORs dur- spectively, were obtained from Jackson ImmunoResearch Laboratories, f/1
3
ing mitosis have already been identified as components of Inc. (West Grove, PA). Peroxidase-conjugated secondary antibodies with 3/2
the rDNA transcription machinery, such as subunits of specificity for human, rabbit, or mouse IgGs were obtained from Amer- /23
5
RNA pol I (28, 29, 40, 51) and the RNA pol I transcription sham France (Les Ulis, France). /12
6
factor UBF (6, 38, 53), or as being essential for rRNA syn- 55
8
thesis, such as DNA topoisomerase I (16). However, the Preparation of Chromosome-associated Protein 3/2
3
characteristics of the mitotic rDNA transcription machin- Extracts for SDS-PA GE 5.p
d
eclreya rly are estsiltl ablished poorly understthoaotd . some subunitsF or example, of the even RNA if it pol was I H14 eLa h. Mitcoetlilsc were cells blocked were inh arvested mitosis by by colchicinem echanical treatment shock. The (0.1 ~.g/ml) procedure for f by gu
e
are localized in NORs, it has never been proven that the used to isolate chromosomes was adapted from Young et al. (52). The st o
RNA pol I complex is maintained during mitosis and asso- cells (at least 98% mitotic cells) were centrifuged at 100 g for 10 min, sus- n 1
cnioatt ed yet with been NORs. investigated Similarly, either the during localization mitosis of or SL1 during has pmpioenln.y daemdin e The at 701bcuelflsf er cells were (15 per mM centrifumgle d Tris-HCl, in 75 mM (100 pH KCI, 8, g, 2 and m10 M minicnu)b atEedD TA, and 0.5 at suspendedm M 37°C EGTA, for 15 in 1 March 2
interphase. Moreover, as for RNA pol I, the presence of 80 mM KC1, 20 mM NaCI, 0.2 mM spermine, 0.5 mM spermidine, and 3 02
3
the SL1 complex during mitosis has never been reported. mM dithiothreitol). Cell membranes were then mechanically disrupted by
addition of 0.25% (vol/vol) Triton X-100 and passage through a G22 nee-
The aim of the present study was to investigate if the
dle until most of the chromosomes appeared scattered as assessed by
rDNA transcription machinery, as it was defined by in
phase microscopy. To discard nonlyzed mitotic ceils and contaminating
vitro reconstituted transcription systems, remains associ- nuclei, the chromosome suspension was centrifuged on a 0.25 M sucrose
ated with NORs during mitosis. Other questions concern cushion in polyamine buffer at 600 g for 5 min, and the supernatant recen-
the respective distribution of RNA pol I and transcription trifuged on a similar sucrose cushion at 34000 g for 30 min. SDS-PAGE
sample buffer (27) was added to the pellet containing the chromosomes.
factors in the different NORs that could explain the vari-
These chromosome extracts were sonicated, boiled for 5 min, and centri-
ability of NOR activity during interphase. In addition, the fuged. The proteins in the supernatant corresponding to cytoplasm were
stability of this association during mitosis and the partition precipitated by 5 vol of cold acetone and kept at -20°C for 1 h. The pre-
between the daughter cells would shed light on the trans- cipitated proteins were collected by centrifugation and solubilized in SDS-
PAGE sample buffer. The cytoplasmic extracts were sonicated, boiled for
mission of the rDNA transcription machinery through cell
5 min, and centrifuged.
cycle. Finally, these results will help to clarify the defini-
tion of "active" contrary to "inactive" NORs at least for
Preparation of Cell, Nuclear, and Nucleolar Protein
the proteins involved in the rDNA transcription. Since the
Extracts for SDS-PA GE
human NORs differ with respect to the number of rDNA
copies and potential activities (9, 10), these are ideal con- All extracts were prepared from exponentially growing HeLa cells. For
The Journal of Cell Biology, Volume 133, 1996 236
whole cell extracts, cells were washed in culture medium without serum min. After washing in PBS, cells were permeabilized with acetone at
and were lyzed in SDS-PAGE sample buffer. -20°C for 3 min.
For nuclear and nucleolar protein extracts, cells were lyzed at 4°C in For immunofluorescence labeling, cells grown as monolayers or chro-
TKM buffer (10 mM Tris-HCl, pH 7.4, 10 mM KC1 and 3 mM MgCI2). Ly- mosome spreads were incubated with sera at RT for 45 min, and the anti-
sis was stopped when the nuclei appeared free of cytoplasmic components bodies were revealed by Texas red-conjugated goat anti-human and/or
as assessed by phase microscopy. The nuclear and nucleolar proteins were FfTC-conjugated goat anti-rabbit antibodies. DNA was visualized with
prepared as previously described (37). The nucleoli were isolated by soni- 4',6-diamidino-2-phenylindole (DAPI). All preparations were mounted
cation of nuclei suspended in TKM buffer and purified by centrifugation with an antifading solution (Citifluor, Canterbury, UK).
on a 0.88 M sucrose cushion in TKM buffer at 1,300 g for 15 min. All steps Fluorescent microscopy was performed using a microscope (DMRB; E.
were performed at 4°C, and all of the solutions contained an antiprotease Leitz, Inc., Rockleigh, N J). Images were photographed using a microscope
cocktail: 1 mM PMSF, 10 ~g/ml aprotinin, 1 ixg/ml pepstatin, and 1 p.g/ml camera system (DMRD; E. Leitz, Inc.). The superimposition of images
leupeptin. The nucleolar proteins were solubilized in SDS-PAGE sample was obtained by scanning micrographs using Ofoto 1.0.1 on Onescanner
buffer. (Apple Computer, Inc., Cupertino, CA). Images were then assembled and
printed using Canvas (Deneba Systems Inc., Miami, FL) and Adobe Pho-
Immunoblotting toshop (Adobe Systems Inc., Mountian View, CA). All computer manipu-
lations were done with a Macintosh Quadra 700 (Apple Computer Inc.).
The proteins were separated by 10% SDS-PAGE using a Protean II cell Images were printed directly from the computer on a printer (ColorEase
(Bio-Rad Laboratories, Richmond, CA). Size standards from 200 to 14 PS Printer; Eastman-Kodak Co., Rochester, NY).
kD (Bio-Rad Laboratories) were included in each gel. The polypeptides
were electrotransferred to reinforced cellulose nitrate membranes (BA-S Assay of RNA Polymerase Activity In Situ
83; Schleicher & SchueU, Dassel, Germany), that were then cut into strips.
The strips were blocked by incubation for 1 h in PBS containing 5% (wt/ The assay was performed on HeLa cells grown as monolayers essentially
vol) dried milk and 0.05% (vol/vol) Tween-20 and incubated with the sera as described (32). Cells were incubated in the assay solution (100 mM
for 2 h in the same buffer. They were then washed three times with PBS Tris-HC1, pH 7.9, 12 mM 2-mercaptoethanol, 150 mM sucrose, 0.6 mM
containing 5% (wt/vol) dried milk and 0.5% (vol/vol) Tween-20 and incu- ATP, CTP, and GTP, 0.12 mM bromo--uridine 5'-triphosphate (BrUTP), D
bated for 1 h in the presence of HRP-labeled second antibodies. After and 12 mM MgC12) at 37°C for 15 min. The reaction was terminated by ow
several rounds of washing, the HRP activity was detected using the en- rinsing the slides in PBS and postfixing with 2% (wt/vol) formaldehyde in nlo
hanced chemiluminescence kit (Amersham France) and recorded on x-ray PBS at RT for 15 min followed by permeabilization with acetone at -20°C ade
film (Fuji Photo Film Co., Ltd., Tokyo, Japan). f(o49r) 3 by rain. immunofluorescBernUceT P incorporation labeling was then using detected a mouse as anptrie-vbiroousmloyd eoxyuri- described d from
dine mAb revealed by FITC-conjugated goat anti-mouse antibodies. In h
Immunoprecipitation HeLa cells were metabolically labeled with 35Smethionine (Tran35S - sveoamleed experimenbtys , Texas red-conjugated UBF was simultaneously goat anti-human detected antibodies. using serum A17 re- ttp://rupre
label; ICN Biomedicals, Orsay, France). Interphase cells were scraped off noC focal Laser Scanning ypocsorciM ss.o
in ice-cold PBS after eliminating mitotic cells by mechanical shock. Mi- rg
thtEwooetatDriireccT v eAs,t ed ceclellsl s, sonicated 1 webrye mMo btained mechanical eixotdroaacactneedtd a mide, by centrifuged colchicine shocink 50 mMa nda nd at treatmweTarnsitsh100,000 - eHdC1 %l , (vol/vol) in g (0.1 pHP BS. for 8, p.g/Tml)r it1 on The 500 h. The imnMt erphaosvXe-e1 0r0.n ight, supNeaCrI,n atants Extracts o5 r were mM mi- C(IAaonBncnn.idf, o o-cRaaid argona Garden helium-neon laisoLenar b oratories), City, laser scanning NY) adijouns ted elqasueirp ped mimcoruosnctopeyd to adjusted 488 with nmo n wast oa wa as ×60 54m3 icroscope perfoursemde d objnecmti ve for for the Texas( Optipuhso(itpn lga n fluorescein red. apoa;n TMhReCII -; 600 NA sNiignkaol,n emit- 1.4). /jcb/article-pdf/133
were adjusted to 50 mM Tris-HCl, pH 8, 2 mg/ml BSA, 150 mM NaCl, 5 ted light was separated by a dichroic mirror (DR565; Bio-Rad Laborato- /2/2
mM EDTA, 1 mM iodoacetamide, and 0.2% (vol/vol) Triton X-100 and 3
were Before used imimnu noprecipitationt,h e immunoprecipitation antibodies assays. were cross-linked to protein A riine s), front and of a t(h54e0D F30; photomultiplier Bio-Rad Labcoorllaetctoirnig es) the long fluorescein pass filter emission. was placed For 5/1265
each optical section, double fluorescent images were acquired in two 5
co(Pviaelrecnet ly Chemical linked Co.t,o agarose Rockford, using IL). the Protein ImmunoPure A-antibody IgG cOomrpileenxteast ion were Kit psaysssetse: m wafs luorescein adjusted first, to allow Texas a fireled d second.d epth of The ~0.5 pinhole ixm, correspondionfg the confocal to 83/235
incubated with extracts at 4°C for 2 h. Immunoprecipitates were washed .p
tEwiDceT A, in 1 50 mM mM iodoacetamide, Tris-HCl, pH and 8, 2 1% mg/ml (vol/vol) BSA, Triton 150 mM X-100, NaCt, once 5 in mM 50 tghree en increment and red sbiegntawles en was two collected adjacent for sections. each specimen A focal searnide s then including processed the df by g
m6.8. aMnd 1E% Tarcish-H C1, (vol/vol) immunoprecipitation pH Triton 8, 500 X-100, mM NaC1, and reaction finall5 y mM was twicEe DTA, carried in 100 1 mM out mM iodoacetaumsiidneg, Tr is-HC1, extracts pH teloaa tcthe r produce of case, the steresoi-ngpolapeit risc al composite section with or a imagesti lt extended of -0.6/(e+x0t.e6n ded focus imawgeerse focus). generated, were For mecorlgoecdal.i zation, combining In the uest on 11
a high spatial resolution with the observation of the colocalization in the M
fmruonme 500,000 sera (A18 cells. and ImmunopreVcilpilta)t es and human nonautoiombmtuainnee d using serhuum man (C) autoim- were cell volume. arch
analyzed by 10% SDS-PAGE followed by autoradiography. 20
2
Results 3
Cell Culture and Immunofluorescence Labeling
Antibodies against the rDNA Transcription Machinery
HeLa cells were cultured in Eagle's minimum essential medium (Flow
Laboratories, Paris, France) supplemented with 10% (vol/vol) FCS. Cells The characterization of sera directed against the transcrip-
were seeded three times a week, and extracts were prepared 24 h after tion machinery was carried out using nucleolar extracts
seeding from exponentially growing cells.
prepared from actively growing HeLa cells and partially
For immunolocalization, HeLa cells were grown as monolayers on glass
slides. The glass slides were previously treated with 5% (wt/vol) KOH in purified RNA pol I complex. The human autoimmune se-
methanol for 12 h, rinsed in water, treated with 0.01 N HCI for 1 h, washed rum A18 recognized two polypeptides in HeLa cell nucle-
in distilled water and ethanol, and sterilized. Cell monolayers were rinsed olar extracts (Fig. 1 A). The molecular masses of these two
in PBS and fixed with 80% ethanol at -20°C for 8 min or with 2% (wt/vol)
polypeptides were 200 and 120 kD. Using partially puri-
formaldehyde in PBS at RT for 20 min followed by washing in PBS and
fied mouse RNA pol I complex (43, 44), these two
permeabilization with acetone at -20°C for 3 rain.
HeLa chromosome spreading for immunolocalization was carried out polypeptides were identified as the two largest subunits of
as follows: HeLa cells were blocked in mitosis by incubation in 0.1 ~g/ml RNA polymerase I (Fig. 1 B). A18 also recognized the two
of colcemid for 2 h. Mitotic cells harvested by mechanical shock were cen-
largest subunits of the purified yeast RNA pol I (data not
trifuged at 100 g for 10 min, suspended at 701 cells per ml in FCS diluted
shown). Similarly, serum Vll that recognized a 60-kD
1:5, and incubated at RT for 25 rain. Cells were then cytocentrifuged on
glass slides and fixed in 2% (wt/vol) formaldehyde in PBS at RT for 20 polypeptide in HeLa cell nucleolar extracts (Fig. 1 A) rec-
Roussel et al. rDNA Transcription Machinery during Mitosis 237
ognized the 62-kD polypeptide corresponding to the third polypeptide could be a proteolytic product of the 60-kD
largest subunit of the partially purified mouse RNA pol I subunit. However, the most striking point is the similarity
complex (Fig. 1 B). Serum A17, previously characterized of the complexes immunoprecipitated. This similarity ar-
as containing anti-UBF antibodies (38) and anti-TBP anti- gues in favor of the fact that both sera are directed against
bodies, were tested on these nucleolar extracts (Fig. 1 A). antigens of the same complex that are most likely different
UBF, appearing as a doublet of 97-94 kD, and TBP were subunits of the RNA pol I complex.
present in the nucleolar extracts. Except for TBP mainly
found in nuclear extracts, UBF and the antigens revealed
Association of the rDNA Transcription Machinery with
by A18 and Vll are concentrated in nucleolar extracts as
Chromosomesd uring Mitosis
illustrated for V11 (Fig. 2).
Immunoblotting seemed to indicate that serum A18 rec- The antibodies were probed on chromosome-associated
ognized the two largest subunits of RNA pol I, and serum protein extracts and cytoplasmic extracts prepared from
Vll recognized the third largest subunit of RNA pol I. To the same number of mitotic cells. The results obtained
confirm the specificity of these sera, immunoprecipitations with serum Vll, anti-UBF, and anti-TBP antibodies are
were carried out anticipating that by using antibodies di- presented (Fig. 3). The 60-kD polypeptide revealed by
rected against one or two RNA pol I subunits, it would be Vll (corresponding to the 60-kD subunit of the RNA pol
possible to immunoprecipitate the entire RNA pol I com- I) was found only in the chromosome-associated protein
plex and a similar complex with both sera. At least with re- extracts (Fig. 3, lane a), but not in the cytoplasmic extracts
spect to the major bands immunoprecipitated using both (Fig. 3, lane b). Likewise, UBF was observed in the chro-
the A18 and Vll sera, the molecular masses correlated mosome-associated protein extracts (Fig. 3, lane c). A very
well with those of subunits of the RNA pol I complex typi- weak amount of UBF, only detected when the film was D
o
cal of mammalian cells (43). The major bands immunopre- overexposed, was present in the cytoplasmic extracts (not w
n
cipitated from interphase cell extracts using A18 and Vll visible in lane d), most likely due to release during chro- loa
d
e
were 190-, 120-, 52-, 43-, 29-, and 17-kD polypeptides (Fig. mosome isolation. Similarly TBP (Fig. 3, lane e) was d
1 C). Minor bands were observed of 185, 170, 155, and 135 present in the chromosome-associated protein extracts. from
h
kD, corresponding most likely to proteolytic products of The presence of one subunit of the RNA pol I that asso- ttp
the 190-kD polypeptide. These cleavages were also ob- ciated with chromosomes during mitosis did not prove the ://ru
p
served in enriched mouse RNA pol I extracts (data not association of all the subunits of the complex. Therefore, re
shown), in favor of the instability of the highest RNA pol I immunoprecipitations were carried out to investigate the ss.o
rg
subunit. Serum Vll recognized a 60-kD polypeptide in hu- fate of the RNA pol I complex during mitosis. In mitotic /jcb
ma an complex, nucleolar as did extracts A18 in (Fig. which 1 A) no and 60 kD immunoprecipitated was found (Fig. 1 ce(lFlig . extracts, 4 b) and similar Vll (Fig. results 4 c). were The obtaisnaemde major using bands sera were A18 /article-p
d
C). As the 52-kD polypeptide immunoprecipitated with observed as in interphase but with two additional bands of f/1
3
both sera is more abundant with Vll than with A18, this 75 and 41 kD. The fact that similar patterns were obtained 3/2
/2
3
5
/1
erugiF .1 Identification of 265
5
antigens by immunoblotting 83
and immunoprecipitation. /23
5
(A) HeLa nucleolar protein .pd
belxottrsa cts by retvheea led sera A18 on Western (lane a, f by gu
e
revealing two polypeptides st o
n
of 002 and 021 kD), Vll 1
1
(lane b, revealing a 60-kD M
a
polypeptide), anti-UBF anti- rch
bodies (lane c), and by anti- 20
2
TBP antibodies (lane d). 3
Numbers indicate the molec-
ular masses in kD of proteins
revealed with the sera A18
and Vll. (B) Partially puri-
fied mouse RNA pol I re-
vealed on Western blots by
A18 (lane a) and Vll (lane
b), recognizing the two larg-
est subunits and the third
largest subunit of mouse
RNA pol I, respectively. (C)
Immunoprecipitation assays
carried out using control se-
rum (C; lane a) and anti-
RNA pol I antibodies (A18
and Vll; lanes b and c) on in-
terphase HeLa extracts. cell
ehT lanruoJ of lleC ,ygoloiB .331 emuloV 6991 832
erugiF .2 Localization of antigens dur-
ing interphase by immunoblotting. The
same amount of nucleolar (lane a), nu-
clear (lane b), and cell protein extracts
(lane c) revealed on Western blots with
Vll. The position of the 60-kD erugiF .4 Immunoprecipita-
polypeptide indicated. is tion assays carried out using
control serum (C; lane a) and
anti-RNA pol I antibodies
using mitotic cell extracts and interphase cell extracts (A18 and Vll; lanes b and c)
on mitotic HeLa extracts. cell
proves that the RNA pol I complex present during inter-
Proteins specifically immu-
phase is also present during mitosis. We conclude that the
noprecipitated are indicated
subunits composing the RNA pol I complex remain associ-
by their molecular masses in
ated during mitosis. kD. The asterisks point to Do
w
specifically immunoprecipi- n
Localization of the rDNA Transcription Machinery tated bands only observed load
e
during Mitosis with mitotic extracts. cell d fro
m
In the chromosome-associated protein extracts of HeLa h
mitotic cells, we observed the presence of the RNA pol I As illustrated for early metaphase (Fig. 5 b), labeling ttp://ru
subunits, TBP, and UBF by immunoblotting. This is com- p
corresponding to TAFIll0 appeared as discrete spots of re
patible with the hypothesis that the rDNA transcription variable sizes in association with chromosomes (compare ss.o
mTmdiaoetcf ohitniienc ery test if thicsel ls they remahiynpost hesist,h e are majcoolro calized. associated we components proceeded We to firstr DNAs of to detected this immunodetecdtur ing machinery each com- mitosis. toin 5F,ti gh. e c-e) same5, a showed and HeLa b). a mitTothiec similar examination cells localization (illustrated of oUfB F TAFIflorl 0 and telophase, TAFIll0 and UBF Fig. in rg/jcb/article-pd
ponent individually. As expected by the results of immu- and similar variations in the size of the spots. Indeed, simi- f/13
noblotting, RNA pol I, UBF, and TBP remained associ- lsamra ller patterns were spots revealed obtained with ainn ti-TAFill0 both cases: antiboditehse larger (Fig. and 3/2/23
ated with the condensed chromosomes at all phases of the 5 d) corresponded respectively to the larger and smaller 5/12
mitosis (data not shown). We also investigated the possi- spots revealed with anti-UBF antibodies (Fig. 5 e). Similar 655
ble colocalization of the transcription factors SL1 and 83
results were obtained whatever the mitotic stage examined /2
UBF that, in addition to RNA pol I, are necessary and suf- 35
(data not shown). .p
ficient to promote rDNA transcription in human cells. d
The localization of TBP during mitosis in HeLa cells is f b
Since SL1 is composed of TBP, TAFIll0, TAFI63, and illustrated for anaphase (Fig. 5 g) and for metaphase (Fig. y gu
cTAaFrIr4i8e,d out the using localization anti-TBP of and SL1 anti-TAFi1i1n 0 mitotic HeLa antibodies. cells was 5 j). In both anaphase and metaphase, as was the case at all est on
stages of mitosis (data not shown), part of TBP appeared 1
1
in discrete spots in association with chromosomes. As ex- M
a
pected, part of TBP was not associated with chromosomes rch
erugiF .3 Localization of an- but was dispersed in the cytoplasm (Fig. 5 g). The localiza- 202
tigens during mitosis by im- 3
tion of both TBP and UBF in the same mitotic cells (Fig. 5,
munoblotting. Comparable
f-h, and i-k) showed that at all mitotic stages, the part of
amounts of chromosome
(lanes a, c and e) or cytoplas- TBP appearing in spots in association with chromosomes
mic extracts (lanes b and d) was localized in the same position as UBF. As reported for
revealed on Western blots TAFIll0, the spots varied in size, but these variations
with Vll (lanes a and b), were similar for TBP and for UBF (compare Fig. 5, g and
anti-UBF antibodies (lanes c h, and j and k). The similar localization of TAFIll0 and
and d), and anti-TBP anti- TBP indicates that the transcription factor SL1 is most
bodies (lane e). The 60-kD
probably maintained during mitosis and localized in NORs
polypeptide revealed by Vll
during mitosis as is UBF (38). The fact that the size of the
(lane a) and UBF (lane c) si
spots varied in a correlated manner for UBF, TAFIll0,
present in chromosome ex-
and TBP indicates that the amounts of UBF, TAFIll0,
tracts and absent from cyto-
plasmic extracts (lanes b and and TBP vary in the same proportions between different
d). TBP (lane e) is partially positive NORs.
present in chromosome ex- To validate these observations, the colocalization of
tracts. these factors in the volume of mitotic cells was established
Roussel et al. rDNA Transcription Machinery during Mitosis 239
by confocal microscopy. The fluorescent patterns ob-
served in the same mitotic HeLa cells for RNA pol I (us-
ing Vll and A18) and TBP (Fig. 6) or for UBF and TBP
(data not shown) were identical from prophase to telo-
phase. The same number of spots, and spots varying in size
in the same proportions, were observed for both labelings
in the two cases using extended focus images obtained af-
ter three-dimensional reconstruction of the serial optical
sections (Fig. 6; illustrated for anaphase, metaphase, and
interphase). The superimposition of RNA pol I and TBP
labelings recorded in the same optical section indicated
strict colocalization of RNA pol I and TBP in mitosis as
well as in the nucleolus during interphase (Fig. 6, C, C',
and C", and D, D', and D"). In extended focus images six
chromosomes were scored positive for RNA pol I colocal-
ized with TBP (Fig. 6, A, A', and A") and also for UBF
colocalized with TBP (data not shown).
Because UBF is known to remain associated with only
some NORs during mitosis (6, 38, 53), these results indi-
cated that UBF, RNA pol I, and SL1 (verified for TBP
and TAFIll0) were associated with the same NORs dur- D
o
ing mitosis in HeLa cells. wn
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e
Different Types of NORs d
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In human cells there are 10 NOR-bearing chromosomes, h
but the rDNA transcription machinery remains associated ttp://ru
in variable amounts with only six of them. We were inter- p
re
ested in identifying the positive NOR-bearing chromo- ss.o
sgiovmeens cell to detliene.r mine if they constitute a characteristic in a rg/jcb/a
Chromosome spreads prepared from HeLa cells blocked rticle
in mitosis made it possible to recognize acrocentric chro- -pd
mosomes (Fig. 7) that were previously proved to be the f/13
3
NOR-bearing chromosomes in human cells. Immunolabel- /2/2
3
ings of UBF and RNA pol I (Fig. 7) obtained on HeLa 5/1
2
chromosome spreads were similar. In both cases, six spots 65
5
or double-spots were visible. These proteins were not 83
/2
equally distributed between NORs. Some NOR-bearing 35
.p
chromosomes scored negative, and the intensity of the la- d
f b
beling of the six positive NORs differed. Interestingly, y g
u
UsixB F different and RNA positive pol I NORvasr.i ed in the same proportions in the est on
1
In this HeLa cell line, the labeling pattern was the same 1 M
in the different chromosome spreads observed using anti- arch
bodies directed against UBF or the RNA pol I subunits 20
2
(Fig. 7). There were always six positive NOR-bearing 3
chromosomes, three small acrocentric chromosomes (chro-
These images are reminiscent of background often associated us-
ing rabbit antibodies. (f-h) Anaphase HeLa cell labeled with
anti-TBP (g) and anti-UBF antibodies (h) and DNA stained by
erugiF .5 Localization of UBF and TAFIll0, and of UBF and DAPI (]). TBP labeling (g) appears as discrete spots in associa-
TBP, in the same mitotic cells. (a and b) Early metaphase HeLa tion with chromosomes (compare f and g) and as labeling dis-
cell labeled with anti-TAFill0 antibodies. The labeling appears persed in the cytoplasm. The discrete spots revealed by anti-TBP
in discrete spots (b) in association with chromosomes revealed by coincide exactly with the spots revealed by anti-UBF antibodies
DAPI (a). (c-e) Telophase HeLa cell labeled both with anti- .)h( (i-k) Metaphase HeLa cell labeled with anti-TBP (/'a)n d
TAFIll0 (d) and anti-UBF antibodies (e) and DNA stained by with anti-UBF antibodies (k) and DNA stained by DAPI (i). A
DAPI (c). The colocalization of UBF and TAFIll0 si shown by part of the TBP labeling appears as discrete spots (j) in associa-
identical labeling patterns obtained with both antibodies. The tion with chromosomes (i). The spots revealed with anti-TBP an-
weak cytoplasmic labeling observed with anti-TAFill0 antibod- tibodies (j) coincide exactly with the spots revealed with anti-
ies is homogeneous and visible in mitotic and interphase cells. UBF antibodies (k). Bar, 01 .m~p
ehT lanruoJ of lleC ,ygoloiB emuloV .331 6991 042
mosomes 21 or 22), and three large acrocentric chromo- Discussion
somes (chromosomes 13, 14, or 15). Thus, for a pair of ho-
In vitro studies made it possible to define the rDNA tran-
mologous NOR-bearing chromosomes, the first chromosome
scription machinery necessary to obtain accurate in vitro
could be positive, whereas the second one could be nega-
rDNA transcription. The rDNA transcription machinery
tive. Moreover, when the labeling of a positive chromo-
is composed in humans of the RNA pol I complex, the
some appeared as a double-spot (i.e., when the two spots
transcription factor UBF, and the transcription factor SL1,
corresponding to the two chromatids were not coalesced),
composed of TBP and three TAFIs: TAF1110, TAFt63,
both spots composing the double-spot were of equal inten-
and TAFI48 (2, 3, 7, 24). It was recently proven that the
sity. Therefore, the amount of protein associated with
three TAF~s together with TBP are necessary and suffi-
both chromatids of the same chromosome seemed to be
cient to reconstitute a transcriptionally active SL1 com-
equivalent. This explained the equal partition of UBF and
plex (54). In vivo, expression of the rDNAs is dependent
RNA pol I and consequently of the rDNA transcription
on cell cycle and cell growth and takes place in the nucle-
machinery during anaphase when the separation of chro-
oli. During mitosis, rDNA transcription is arrested, but the
matids occurs.
early resumption of its transcription at telophase raises the
Keeping in mind that the different components of the
question of the fate of the rDNA transcription machinery
rDNA transcription machinery are colocalized during mi-
during mitosis. As the NORs are the sites where rDNA
tosis, we can conclude that RNA pol I, UBF, and also SL1
transcription is activated in telophase (4, 5, 13, 25, 33), we
are associated with the same NORs in a given cell and al-
focused our attention on the proteins associated with the
ways with the same NORs in this HeLa cell line, and that
NORs during mitosis to investigate the transitions be-
they vary in the same proportions in the different positive
tween mitosis and interphase.
NORs. D
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lo
gnitegraT of the rDNA noitpircsnarT Machinery ad
The noitpircsnarT Activity of evitisoP NORs e
d
gnirud sisotiM fro
BNeOcRasus e and the requires reformation rDNA of transncurcilepotliio n, occurs we in could telophase predict at For accurate rDNA transcription, it seems reasonable to m http
that NORs are the sites where the rDNA transcription postulate that the same partners interact in vivo as in vitro. ://rup
vmaancthaignee ry of is the activated. fact that To RNA verify pol this activities point, mawye took be pre- ad- vIinv o this transcription case, the minimal is active transcriptioRn NA pol I mcaocmhpilneexr y and for the in ress.org
served in situ in cells (32). During interphase the RNA pol specific RNA pol I transcription factors UBF and SL1. /jcb
I activity (insensitive to a-amanitin) occurring in nucleoli mitosis RNA in pol different I has already mammalian been localizecedl ls (14, into 17, 18, NORs 23, 40, during 51). /article
is particularly well detected in the conditions used (32). At -p
the transition interphase/mitosis, the RNA pol I activity The mammalian subunits composing the RNA pol I com- df/1
3
plex are not as well characterized as the yeast subunits 3
stopped rapidly before the destructuring of nucleoli (data /2
not shown). During mitosis, no RNA pol activity was ob- (45), but recent progress indicated a similar number of /235
served as illustrated for prometaphase HeLa cells (Fig. 8, subunits in mouse and yeast RNA pol I complex (46). In /126
C and C'), except for during telophase (Fig. 8, A, A', A", the present study, we present evidence for the presence of 558
B, B', C, and C'). During telophase RNA pol I activity was the three largest RNA pol I subunits in the NORs by im- 3/23
derepressed as evidenced by the incorporation of BrUTP munolocalization, and by immunoprecipitation we demon- 5.pd
paaprpeea ring Fig. 8, in A', spots A", in A, association B', B, C', with and chromC)o. somes The fluorescent (com- splterxaets e that (see mitotibce low forc ell the extracts discussion contain of thReN A mitotic pol I RNAco m- f by gue
patterns observed in the same telophase HeLa cells for pol I complex). st o
n
BrUTP incorporation and for UBF (Fig. 8, B', B", C', and RNA pol I transcription factors were also associated 11
with the mitotic chromosomes as already reported for the M
C") showed that both labelings were superimposable. It is a
noteworthy that the unusual background obtained using UBF (6, 35, 38, 53). In addition, we proved that SL1 (as rch 2
anti-UBF antibodies was most likely due to the procedure verified for TBP and TAFIll0) is most likely maintained 023
during mitosis and also remains associated with NORs.
used to detect RNA pol activity. The localization of UBF,
Moreover, all of the rDNA transcription machinery as de-
and consequently of the rDNA transcription machinery,
fined in vitro is colocalized at NORs during mitosis: RNA
and the detection of RNA pol I activity made it possible to
pol I, UBF, and SL1 are associated with the same NORs
observe that the rRNA transcription machinery was dere-
whatever the mitotic stage investigated. We failed to ob-
pressed at telophase at NORs. Because up to six spots cor-
tain labeling with TAFI48 antibodies in mitosis or in inter-
responding to sites of RNA pol I activity could be ob-
phase. Because we cannot suspect the absence of TAFI48
served in telophase HeLa cells (Fig. 8, A' and A"), the
in active rDNA transcription machinery in interphase, this
rDNA transcription machinery associated with the six
result is most likely due to inaccessibility of the epitopes or
NORs seemed to be derepressed simultaneously. More-
modifications induced by fixing the cells.
over, since the intensity of the labelings corresponding to
the sites of BrUTP incorporation varied in the same pro-
The rDNA noitpircsnarT Machinery Is dezilacoloC in
portion as the size of the spots corresponding to UBF
Active NORs and Absent from Inactive NORs
(compare Fig. 8, B' and B", and C' and C"), the level of
transcription activity seemed to be directly related to the The active NORs were previously defined as NORs in
amount of rDNA transcription machinery present in the which potentially active rDNAs are located. This implies
NOR. that these genes can be transcribed during interphase as
Roussel e! a. rDNA Transcription Machinery during Mitosis 241
D
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The Journal of Cell Biology. Volume 133, 1996 242
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Figure .7 Localization of UBF and RNA pol I by immunofluorescence on HeLa chromosome spreads. HeLa chromosome spreads st o
stained by DAPI and labeling obtained with anti-UBF antibodies (A and B) or with Vll (C) were superimposed. The overall picture n 1
1
(A) shows that the labeling pattern is the same when comparing one chromosome spread to another. The comparison between labeling M
a
obtained with anti-UBF (B) and anti-RNA pol I antibodies (C) demonstrates that the same six acrocentric chromosomes (NOR-bearing rch
chromosomes in humans) are labeled. Both labelings vary in the same proportions in the six positive chromosomes. The acrocentric 20
2
chromosomes devoid of either labeling are indicated by arrowheads (B and C). Bar, 5 .mxt 3
opposed to the other rDNA copies that cannot be tran- NOR proteins (30, 31). The Ag-NOR proteins are easily
scribed. As demonstrated by experiments using interspe- visualized in situ by cytological silver staining (15, 22). We
cific somatic cell hybrids, NORs bearing potentially active previously reported that in mitotic HeLa cells, UBF is only
rDNAs are characterized by their association with Ag- localized in active NORs where Ag-NOR proteins are also
Figure6. (cid:127)ac(cid:127)fn(cid:127)C r(cid:127)sa(cid:127) scanning yp(cid:127)cs(cid:127)rcim dem(cid:127)nstrating n(cid:127)itazi(cid:127)ac(cid:127)(cid:127)(cid:127)c f(cid:127) RNA (cid:127)(cid:127)p (cid:127) and TBP during mit(cid:127)sis. (A(cid:127)A (cid:127)' and A '') The
anaphase HeLa cell was labeled with Vll and anti-TBP antibodies. (A) Extended focus of labeling obtained with Vll. (A') Extended
focus of labeling obtained with anti-TBP antibodies. (A") Merged image of both labelings. Yellow color indicates colocalization of
RNA pol I and TBP present in six spots in association with chromosomes of each daughter cell. (B-D) Metaphase and interphase HeLa
cells were labeled with Vll and anti-TBP antibodies. (B) Three-dimensional stereoscopic projections of optical sections ofb oth label-
ings showing the colocalization of RNA pol I and TBP in the cell volume. C, C', and C", and D, D', and D" show the labeling obtained
with Vll (C and D) and anti-TBP antibodies (C' and D') in two successive optical sections. (C") corresponds to the merged image of
both labelings (shown in C and C') recorded in the first optical section, and D" corresponds to the merged image of both labelings
(shown in D and D') recorded in the second optical section. Yellow color indicates strict colocalization of both labelings. Bar, 10 .mxt
lessuoR te .la ANDr noitpircsnarT yrenihcaM gnirud sisotiM 342
erugiF .8 Detection of RNA
pol I activity in mitotic HeLa
ceils. (A, A', A") The telo-
phase HeLa was cell processed
to detect the sites of BrUTP
incorporation. (A) DNA stain-
ing. (A' and A") Two differ-
ently focused images to allow
the observation of the six sites
of BrUTP incorporation in as-
sociation with the two groups
of chromosomes revealed by
DAPI (A). (B, B', B") The te-
lophase HeLa cell was pro- D
o
w
cessed to detect simulta- n
lo
neously BrUTP incorporation ad
e
(anBd ') UBF. (B) BrUTP DNA incorporation. staining. d from
(B") UBF labeling. (C, C', C") http
Telophase and prometaphase ://ru
HeLa cells were processed to pre
detect simultaneously BrUTP ss.o
DinNcoArp oration staining allowing recog- and UBF. (C) rg/jcb/a
nition of the telophase (t) and rticle
the prometaphase cell (p). (C') -p
d
BrUTP incorporation. (C") f/1
3
UBF labeling. The arrows 3/2
point to the labeling observed /23
5
for UBF (C") and the absence /1
2
6
of BrUTP incorporation (C') 5
5
8
in the prometaphase cell. Bar, 3
/2
01 .m~p 35
.p
d
f b
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localized (38). These results are corroborated by the iden- only with the active NORs during mitosis (i.e., six active 1
1
tification of the largest subunit of RNA pol I and UBF as NORs in the HeLa cell line used). In mitotic cells and Ma
Ag-NOR proteins (36). Considering the published results chromosome spreads, the number of positive chromo- rch 2
0
(9, 10, 30, 31) and the results obtained in this study con- somes are always the same in a given cell line. This charac- 2
3
cerning the colocalization of RNA pol I, UBF, and SL1, teristic has also been reported for Ag-NOR proteins (47).
we may conclude that the rDNA transcription machinery In addition, the amount of NOR-associated proteins varies
remains associated only with active NORs during mitosis. between different active NORs in human cells, and this
In the inactive NORs, the rDNAs are not associated with variation correlates with NOR activity during interphase
RNA pol I, UBF, or SL1. We do not know what prevents as proposed for Ag-NOR proteins (1) and for UBF (38).
such associations. However, the level of methylation of the As shown for UBF (38, 53), the rDNA transcription ma-
rDNAs could at least be partly responsible, since high lev- chinery is not equally distributed between active NORs.
els of rDNA methylation are correlated with inactive Indeed the intensity of the labeling of positive NORs dif-
rDNAs (9, 10). The presence of a repressor inhibiting such fered. Double immunofluorescence labeling experiments
associations can also be proposed, but this hypothesis is showed that the different components of the rDNA tran-
not presently supported by biological evidence. scription machinery vary in the same proportions in differ-
ent positive NORs, suggesting a stoichiometric association
of these different components. By analogy with Ag-NOR
The rDNA Transcription Machinery Is Not Randomly
proteins for which the amount in a given NOR is related to
Distributed in Different Active NORs
the number of potentially active rDNA copies in this NOR
The rDNA transcription machinery remains associated (10, 48), we propose that the amount of each component
The Journal of Cell BioloVgoyl,u me 133, 1996 244