The fission yeast rpa17+ gene encodes a functional homolog of AC19, a subunit of RNA polymerases I and III of Saccharomyces cerevisiae

Eukaryotic RNA polymerases I and III consist of multiple subunits. Each of these enzymes includes two distinct and evolutionarily conserved subunits called alpha-related subunits which are shared only by polymerases I and III. The alpha-related subunits show limited homology with the alpha-subunit of prokaryotic RNA polymerase. To gain further insight into the structure and function of alpha-related subunits, we cloned and characterized a gene from Schizosaccharomyces pombe that encodes a protein of 17 kDa which can functionally replace AC19 - an alpha-related subunit of RNA polymerases I and III of Saccharomyces cerevisiae - and was thus named rpa17+. RPA17 has 125 amino acids and shows 63% identity to AC19 over a 108-residue stretch, whereas the N-terminal regions of the two proteins are highly divergent. Disruption of rpa17+ shows that the gene is essential for cell growth. Sequence comparison with other alpha-related subunits from different species showed that RPA17 contains an 81-amino acid block that is evolutionarily conserved. Deletion analysis of the N- and C-terminal regions of RPA17 and AC19 confirms that the 81-amino acid block is important for the function of the alpha-related subunits.

Sequence divergence of the RNA polymerase shared subunit ABC14.5 (Rpb8) selectively affects RNA polymerase III assembly in Saccharomyces cerevisiae

ABC14.5 (Rpb8) is a eukaryotic subunit common to all three nuclear RNA polymerases. In Saccharomyces cerevisiae, ABC14.5 (Rpb8) is essential for cell viability, however its function remains unknown. We have cloned and characterised the Schizosaccharomyces pombe rpb8(+) cDNA. We found that S.pombe rpb8, unlike the similarly diverged human orthologue, cannot substitute for S.cerevisiae ABC14. 5 in vivo. To obtain information on the function of this RNA polymerase shared subunit we have used S.pombe rpb8 as a naturally altered molecule in heterologous expression assays in S.cerevisiae. Amino acid residue differences within the 67 N-terminal residues contribute to the functional distinction of the two yeast orthologues in S.cerevisiae. Overexpression of the S.cerevisiae largest subunit of RNA polymerase III C160 (Rpc1) allows S.pombe rpb8 to functionally replace ABC14.5 in S.cerevisiae, suggesting a specific genetic interaction between the S.cerevisiae ABC14.5 (Rpb8) and C160 subunits. We provide further molecular and biochemical evidence showing that the heterologously expressed S.pombe rpb8 molecule selectively affects RNApolymerase III but not RNA polymerase I complex assembly. We also report the identification of a S.cerevisiae ABC14.5-G120D mutant which affects RNA polymerase III.

Regulation of RNA polymerase I transcription in response to F9 embryonal carcinoma stem cell differentiation

Dramatic changes in the patterns of transcription are a common feature of early development. We have used F9 embryonal carcinoma cells as a model system to study gene regulation during an early stage of murine embryogenesis. We find that transcription by RNA polymerase I decreases when F9 cells differentiate into parietal endoderm. The reduced rate of transcription is associated with a down-regulation of several components of the class I transcription apparatus. The most substantial change involves the essential factor SL1, which is a multisubunit complex that contains the TATA-binding protein and three TATA-binding protein-associated factors (TAFs). The abundance of two of these TAFs, TAFI48 and TAFI95, decreases during F9 cell differentiation. Developmental regulation of a specific class of genes may therefore be achieved through changes in the availability of TAFs.

Instability of pUC19 in Escherichia coli transcription termination factor mutant, rho026

The higher copy number of pUC19, compared to its parent plasmid pBR322, is known to be due to deletion of rop, also known as rom, and to an ori mutation that impedes RNAI:RNAII interaction. pUC19, unlike pBR322, fails to transform E. coli rho mutant rho026 cells. Here we identify two features of pUC19 that contribute to this transformation defect. (1) The pUCori mutation is involved because replacing the pUCori with that of pBR322 restored transformation. (2) Transcription from the lac promoter in pUC19 is important, since deletion or inversion of the promoter or insertion of a transcription terminator (lambdat0) downstream of it restored transformation. Host RNase E activity is responsible for the transformation defect because introduction of an rne-1 allele into rho026 cells suppressed this defect, indicating that RNAI instability due to RNase E is aggravated in the rho026 strain. We suggest that in rho026 cells pUC19 RNAI:RNAII interaction is more impeded than in rho+ cells and Rop/Rom may confer stability by protecting RNAI against RNase E activity because expression of a rom gene inserted into pUC19 restored transformation.

Expansion and contraction of ribosomal DNA repeats in Saccharomyces cerevisiae: requirement of replication fork blocking (Fob1) protein and the role of RNA polymerase I

Saccharomyces cerevisiae carries approximately 150 copies of rDNA in tandem repeats. It was found that the absence of an essential subunit of RNA polymerase I (Pol I) in rpa135 deletion mutants triggers a gradual decrease in rDNA repeat number to about one-half the normal level. Reintroduction of the missing RPA135 gene induced a gradual increase in repeat number back to the normal level. Gene FOB1 was shown to be essential for both the decrease and increase of rDNA repeats. FOB1 was shown previously to be required for replication fork blocking (RFB) activity at RFB site in rDNA and for recombination hot-spot (HOT1) activity. Thus, DNA replication fork blockage appears to stimulate recombination and play an essential role in rDNA expansion/contraction and sequence homogenization, and possibly, in the instability of repeated sequences in general. RNA Pol I, on the other hand, appears to control repeat numbers, perhaps by stabilizing rDNA with the normal repeat numbers as a stable nucleolar structure.

Reconstitution of yeast RNA polymerase I transcription in vitro from purified components. TATA-binding protein is not required for basal transcription

Five purified protein components, RNA polymerase I, Rrn3p, core factor, TBP (TATA-binding protein), and upstream activation factor, are sufficient for high level transcription in vitro from the Saccharomyces cerevisiae rDNA promoter. Rrn3p and pol I form a complex in solution that is active in specific initiation. Three protein components, pol I, Rrn3p, and core factor, and promoter sequence to -38, suffice for basal transcription. Unlike pol II and pol III, yeast pol I basal transcription does not require TBP. Instead, TBP, upstream activation factor, and the upstream element of the promoter together stimulate pol I basal transcription to a fully activated level. The role of TBP in pol I transcription is fundamentally different from its role in pol II or pol III transcription.

[Molecular cloning and characteristics of rpc19+ and rpc40+ Schizosaccharomyces pombe genes, coding for common subunits of nuclear RNA polymerase I and III]

Full-length copies of cDNAs of the rpc19+ and rpc40+ genes encoding the common subunits of nuclear RNA polymerases I and III and the corresponding fragments of chromosomes were isolated from genomic and cDNA libraries of Schizosaccharomyces pombe and characterized. It was established that the cloned genes are located on chromosomes III and II of the fission yeast, respectively. The rpc40+ gene lacks introns, and the rpc19+ gene contains two intervening sequences. The comparison of subunits Rpc19 (125 aa; M 13 722 Da; pI 4.51) and Rpc40 (348 aa; M 39 141 Da; pI 5.40) of Sz. pombe, whose characteristics were deduced from the sequences of their cDNAs, with the orthologous components of other eukaryotes allowed the most conserved structure-functional domains of these proteins to be identified.

Regulation of RNA polymerase I-dependent promoters by the hepatitis B virus X protein via activated Ras and TATA-binding protein

The hepatitis B virus (HBV) X protein is essential for viral infectivity, and evidence indicates that it is a strong contributor to HBV-mediated oncogenesis. X has been shown to transactivate a wide variety of RNA polymerase (Pol) II-dependent, as well as RNA Pol III-dependent, promoters. In this study, we have investigated the possibility that X modulates RNA Pol I-dependent rRNA transcription. In both human hepatoma Huh7 and Drosophila Schneider S2 cell lines, X expression stimulated rRNA promoter activity. Extracts prepared from X-expressing cells stably transfected with an X gene also exhibited an increased ability to transcribe the rRNA promoter. The mechanism for X transactivation was examined by determining whether this regulatory event was dependent on Ras activation and increased TATA-binding protein (TBP) levels. Our previous studies have demonstrated that X, and the activation of Ras, produces an increase in the cellular levels of TBP (H.-D. Wang, A. Trivedi, and D. L. Johnson, Mol. Cell. Biol. 17:6838-6846, 1997). Expression of a dominant negative form of Ras blocked the X-mediated induction of the rRNA promoters, whereas expression of a constitutively activated form of Ras mimicked the enhancing effect of X on rRNA promoter activity. When TBP was overexpressed in either Huh7 or S2 cells, a dose-dependent increase in rRNA promoter activity was observed. To analyze whether the increase in TBP was modulating rRNA promoter activity indirectly, by increasing activity of RNA Pol II-dependent promoters, a Drosophila TBP cDNA was constructed with a mutation that eliminated its ability to stimulate RNA Pol II-dependent promoters. Transient expression of wild-type TBP in S2 cells increased the activities of specific RNA Pol I- and Pol II-dependent promoters. Expression of the mutant TBP protein failed to enhance the activity of the RNA Pol II-dependent promoters, yet the protein completely retained its ability to stimulate the rRNA promoter. Furthermore, the addition of recombinant TBP to S2 extracts stimulated rRNA promoter activity in vitro. Together, these results demonstrate that the HBV X protein up-regulates RNA Pol I-dependent promoters via a Ras-activated pathway in two distinct cell lines. The enhanced promoter activity can, at least in part, be attributed to the X- and Ras-mediated increase in cellular TBP, a limiting transcription component.

Mitotic phosphorylation of the TBP-containing factor SL1 represses ribosomal gene transcription

Entry into mitosis is accompanied by a global repression of transcription. To investigate the molecular mechanisms which shut-down rRNA synthesis during mitosis, we have compared RNA polymerase I (Pol I) transcription in extracts from asynchronous and mitotic HeLa cells. We show by several experimental approaches that phosphorylation by cdc2/cyclin B inactivates the TBP-containing factor SL1 and thus abrogates Pol I transcription during mitosis. This finding links the cell's cycle with the transcriptional activity of Pol I and suggests a common mechanism for mitotic silencing of all three classes of nuclear RNA polymerases, i.e. reversible inactivation of the respective TBP-TAF complexes by (a) mitotic kinase(s).

I-PpoI, the endonuclease encoded by the group I intron PpLSU3, is expressed from an RNA polymerase I transcript

PpLSU3, a mobile group I intron in the rRNA genes of Physarum polycephalum, also can home into yeast chromosomal ribosomal DNA (rDNA) (D. E. Muscarella and V. M. Vogt, Mol. Cell. Biol. 13:1023-1033, 1993). By integrating PpLSU3 into the rDNA copies of a yeast strain temperature sensitive for RNA polymerase I, we have shown that the I-PpoI homing endonuclease encoded by PpLSU3 is expressed from an RNA polymerase I transcript. We have also developed a method to integrate mutant forms of PpLSU3 as well as the Tetrahymena intron TtLSU1 into rDNA, by expressing I-PpoI in trans. Analysis of I-PpoI expression levels in these mutants, along with subcellular fractionation of intron RNA, strongly suggests that the full-length excised intron RNA, but not RNAs that are further cleaved, serves as or gives rise to the mRNA.

Mutation of the gene for the second-largest subunit of RNA polymerase I prolongs the period length of the circadian conidiation rhythm in Neurospora crassa

The period length of the circadian conidiation rhythm was examined in a mutant strain of Neurospora crassa, un-18, that is temperature sensitive for mycelial growth. The un-18 mutant showed a temperature-sensitive phenotype with respect to both mycelial growth and the period length of the conidiation rhythm. Below 22 degrees C, the un-18 mutation did not affect the period length, but at temperatures between 22 degrees C and 32 degrees C, the period length of the un-18 mutant was approximately 2 h longer than that of the wild-type strain. The un-18+ gene was cloned and was found to encode the second-largest subunit of RNA polymerase I, which is involved in the synthesis of rRNA. These results indicate that a defect in ribosome synthesis, which must result in a lower rate of protein synthesis, lengthens the period of the circadian conidiation rhythm in Neurospora.

Expression and purification of recombinant hRPABC25, hRPABC17, and hRPABC14.4, three essential subunits of human RNA polymerases I, II, and III

Transcription of eukaryotic genes is performed by RNA polymerases I, II, and III, which synthesize ribosomal, messenger, and transfer RNAs, respectively. Eukaryotic RNA polymerases are large macromolecular complexes composed of multiple subunits. Among these subunits, five are shared by all RNA polymerases and are essential for cell growth and viability. Remarkably, the human common subunits are structurally conserved and functionally interchangeable with their yeast homologues and are believed to play an important role in the assembly of the three transcription complexes. To understand the structure and function of human RNA polymerases, we overexpressed the common subunits hRPABC25, hRPABC17, and hRPABC14.4 as hexahistidine fusions in Escherichia coli. The recombinant proteins were purified using metal-chelate affinity chromatography on Ni-NTA resin and gel filtration. Depending on the subunit, the yield was 5-17 mg of purified recombinant protein per liter of culture medium. The purified proteins were of high quality and sufficient quantity for structural studies, as demonstrated by the successful crystallization of hRPABC17 and hRPABC14.4. The expression and purification of the common subunits hRPABC25, hRPABC17, and hRPABC14. 4 will make possible their structural analysis with X-ray crystallography and nuclear magnetic resonance, providing important insights into the structure and function of the three human RNA polymerases.

Somatic hypermutation in the heavy chain locus correlates with transcription

Three mutant immunoglobulin heavy chain (IgH) insertion mice were generated in which a targeted nonfunctional IgH passenger transgene was either devoid of promoter (pdelta) or was placed under the transcriptional control of either its own RNA polymerase II-dependent IgH promoter (pII) or a RNA polymerase I-dependent promoter (pI). While the transgene mutation-frequency (0.85%) in memory B cells of pI mice was reduced compared to that in pII mice (1.4%), the distribution and the base exchange pattern of point mutations were comparable. In pdelta mice, the mutation frequency was drastically reduced (0.09%). The mutation frequencies correlated with the levels of transgene-specific pre-mRNA expressed in germinal center B cells isolated from the mutant mice.

TTF-I determines the chromatin architecture of the active rDNA promoter

Transcription of ribosomal genes assembled into chromatin requires binding of the transcription termination factor TTF-I to the promoter-proximal terminator T0. To analyze the mechanism of TTF-I-mediated transcriptional activation, we have used mutant templates with altered sequence, polarity and distance of T0 with respect to the transcription start site. Transcription activation by TTF-I is chromatin specific and requires the precise positioning of the terminator relative to the promoter. Whereas termination by TTF-I depends on the correct orientation of a terminator, TTF-I-mediated transcriptional activation is orientation independent. TTF-I can bind to nucleosomal DNA in the absence of enzymatic activities that destabilize nucleosome structure. Chromatin-bound TTF-I synergizes with ATP-dependent cofactors present in extracts of Drosophila embryos and mouse cells to position a nucleosome over the rDNA promoter and the transcription start site. Nucleosome positioning correlates tightly with the activation of rDNA transcription. We suggest that transcriptional activation by TTF-I is a stepwise process involving the creation of a defined promoter architecture and that the positioning of a nucleosome is compatible with, if not a prerequisite for, transcription initiation from rDNA chromatin.

Cloning and characterization of the human RNA polymerase I subunit hRPA40

The cloning of the human RNA polymerase I 40 kDa subunit, and the comparison of its amino acid sequence to other related RNA polymerase subunits are described. The amino acid sequence of hRPA40 has high homology to the mouse RNA polymerase I 40 kDa subunit (93%), to two Arabidopsis thaliana subunits (47%), the yeast RPC40 subunit (46%) and the human RNA polymerase II hRPB33 subunit (40%). Southern blot analysis shows that this gene is single copy and Northern blot analysis indicates that the mRNA of 1.3 kb is expressed in different cell types.

Deficient transcription of subunit RPA 40 of RNA polymerase I and III in heart of rats with neonatal asphyxia

RNA polymerases transcribe nuclear genes for ribosomal RNA thus representing ribosomal biogenesis. RNA polymerase I transcribes class I genes, coding for large ribosomal RNA and is located in the nucleolus. RNA polymerase III transcribes class III genes, those that encode a number of small ribosomal RNA molecules. Both RNA polymerases form ribosomal biogenesis in a concerted action and have a common subunit, RPA40, essential for function and integrity. The aim of our study was to study the influence of hypoxia/asphyxia on transcription of this subunit as deterioration of ribosomal biogenesis may not be compatible with life. To test this hypothesis we used a nonsophisticated model of neonatal asphyxia. Rat pups were exposed to various asphyctic periods up to twenty minutes and heart tissue was taken for the evaluation of mRNA RPA40 levels, pH measurements and histological evaluation of the nucleolus by silver staining. mRNA RPA40 levels gradually decreased with the length of the asphyctic period paralleling the decrease of pH. Silver staining was remarkably decreased at the asphyctic period of 20 minutes. Our findings of decreased transcription of this essential RNA polymerase subunit indicate impairment of the ribosomal RNA synthetizing machinery and the histological findings suggest its structural relevance. This is the first in vivo observation of deteriorated RNA polymerase in asphyxia/hypoxia.

[Structural-functional characteristics of the Schizosaccharomyces pombe rpb8+ gene, coding the subunit of RNA polymerase I-III, specific only for eukaryotes]

A full-length cDNA of the rpb8+ gene encoding a common subunit Rpb8 of nuclear RNA polymerases I-III only specific for Eucarya was isolated from an expression library of the fission yeast Schizosaccharomyces pombe. The primary structure of the corresponding fragment of the Sz. pombe genome was also established. The rpb8+ gene contains two short introns, 59 and 48 bp long. Only short segments of homology were found upon comparing the Rpb8 subunit homologs from various eukaryotic species, and substantial differences exist between the corresponding proteins of unicellular and multicellular organisms. Subunit Rpb8 of Sz. pombe proved to be the smallest one among the known related proteins: it lacks the 21-aa fragment corresponding to amino acids residues 68-88 of the central part of the homologous subunit ABC14.5 of Saccharomyces cerevisiae. Accordingly, subunit Rpb8 of the fission yeast was not capable of substituting in vivo subunit ABC14.5 in nuclear RNA polymerases of the baker's yeast.

DNA protein-interactions at the Saccharomyces cerevisiae 35 S rRNA promoter and in its surrounding region

This study represents a detailed analysis of the structural context of the RNA polymerase I promoter of Saccharomyces cerevisiae. We determined the presence of regularly spaced nucleosomes in the non-transcribed spacer (NTS) and found that five of them have well defined positions. We show that this nucleosome positioning is restricted to the region between the 35 S and 5 S rRNA promoters, beyond which a more delocalized chromatin structure is evident. A more refined analysis detects the DNA-protein interactions on the RNA polymerase I promoter at nucleotide resolution and provides the first in vivo footprints, attributable to factors like REB1, CF, UAF and an additional protection that seems to be sensitive to the topological context. Moreover, when this analysis is extended to different growth media (YPD versus YNB), some of these protections show a growth condition dependent behaviour.

Affinity purification of mammalian RNA polymerase I. Identification of an associated kinase

Overlapping cDNA clones encoding the two largest subunits of rat RNA polymerase I, designated A194 and A127, were isolated from a Reuber hepatoma cDNA library. Analyses of the deduced amino acid sequences revealed that A194 and A127 are the homologues of yeast A190 and A135 and have homology to the beta' and beta subunits of Escherichia coli RNA polymerase I. Antibodies raised against the recombinant A194 and A127 proteins recognized single proteins of approximately 190 and 120 kDa on Western blots of total cellular proteins of mammalian origin. N1S1 cell lines expressing recombinant His-tagged A194 and FLAG-tagged A127 proteins were isolated. These proteins were incorporated into functional RNA polymerase I complexes, and active enzyme, containing FLAG-tagged A127, could be immunopurified to approximately 80% homogeneity in a single chromatographic step over an anti-FLAG affinity column. Immunoprecipitation of A194 from 32P metabolically labeled cells with anti-A194 antiserum demonstrated that this subunit is a phosphoprotein. Incubation of the FLAG affinity-purified RNA polymerase I complex with [gamma-32P]ATP resulted in autophosphorylation of the A194 subunit of RPI, indicating the presence of associated kinase(s). One of these kinases was demonstrated to be CK2, a serine/threonine protein kinase implicated in the regulation of cell growth and proliferation.

The large subunit of basal transcription factor SNAPc is a Myb domain protein that interacts with Oct-1

The human RNA polymerase II and III snRNA promoters have similar enhancers, the distal sequence elements (DSEs), and similar basal promoter elements, the proximal sequence elements (PSEs). The DSE, which contains an octamer motif, binds broadly expressed activator Oct-1. The PSE binds a multiprotein complex referred to as SNAPc or PTF. On DNAs containing both an octamer site and a PSE, Oct-1 and SNAPc bind cooperatively. SNAPc consists of at least four stably associated subunits, SNAP43, SNAP45, SNAP50, and SNAP190. None of the three small subunits, which have all been cloned, can bind to the PSE on their own. Here we report the isolation of cDNAs corresponding to the largest subunit of SNAPc, SNAP190. SNAP190 contains an unusual Myb DNA binding domain consisting of four complete repeats (Ra to Rd) and a half repeat (Rh). A truncated protein consisting of the last two SNAP190 Myb repeats, Rc and Rd, can bind to the PSE, suggesting that the SNAP190 Myb domain contributes to recognition of the PSE by the SNAP complex. SNAP190 is required for snRNA gene transcription by both RNA polymerases II and III and interacts with SNAP45. In addition, SNAP190 interacts with Oct-1. Together, these results suggest that the largest subunit of the SNAP complex is involved in direct recognition of the PSE and is a target for the Oct-1 activator. They also provide an example of a basal transcription factor containing a Myb DNA binding domain.

The A and B forms of plastid DNA-dependent RNA polymerase from mustard (Sinapis alba L.) transcribe the same genes in a different developmental context

Two RNA polymerases, termed A (cp-pol A) and B (cp-pol B), are known to be present in mustard plastids. In vitro, the two enzymes have different requirements for DNA binding, but both bind to, and transcribe from, the same set of chloroplast promoters. The B enzyme is sensitive to rifampicin (Rif), whereas the A enzyme is not. When seedlings were grown in the presence of Rif, RNA pool sizes of the photosynthesis-related plastid genes rbcL and psbA were smaller than in untreated controls, whereas transcripts of the non-photosynthetic genes rps16, trnG, rrn and rpoB remained virtually unaffected by the drug. The Rif inhibition patterns of rbcL and psbA transcripts reflect the relative abundance of the A and B enzymes at different stages and light/dark conditions. These genes can thus be transcribed by either of the two enzymes in vivo, whereas the non-photosynthetic genes are transcribed mostly or exclusively by the A enzyme, or by another Rif-resistant plastid polymerase. Among several nuclear gene transcripts that were tested for Rif inhibition, only those of the RbcS gene family for the plastid-bound small subunit of Rubisco revealed a decrease in pool size, which may imply that mechanisms exist that serve to coordinate patterns of gene expression in the different cellular compartments.

Terminating transcription in eukaryotes: lessons learned from RNA polymerase I

Within the past few years, the genes encoding transcription terminator proteins for RNA polymerase I (pol I) have been cloned from organisms as diverse as yeast and mammals. The availability of terminator proteins has allowed construction of in vitro transcription systems that terminate pol I at the same sites as used in vivo and thus allows study of termination mechanisms. This has resulted in a burst of information concerning pol I termination mechanisms, which this review will attempt to summarize.

Sok antisense RNA from plasmid R1 is functionally inactivated by RNase E and polyadenylated by poly(A) polymerase I

The hok/sok system of plasmid R1, which mediates plasmid stabilization by the killing of plasmid-free cells, codes for two RNA species, Sok antisense RNA and hok mRNA. Sok RNA, which is unstable, inhibits translation of the stable hok mRNA. The 64nt Sok RNA folds into a single stem-loop domain with an 11 nt unstructured 5' domain. The initial recognition reaction between Sok RNA and hok mRNA takes place between the 5' domain and the complementary region in hok mRNA. In this communication we examine the metabolism of Sok antisense RNA. We find that RNase E cleaves the RNA 6nt from its 5' end and that this cleavage initiates Sok RNA decay. The RNase E cleavage occurs in the part of Sok RNA that is responsible for the initial recognition of the target loop in hok mRNA and thus leads to functional inactivation of the antisense. The major RNase E cleavage product (denoted pSok-6) is rapidly degraded by polynucleotide phosphorylase (PNPase). Thus, the RNase E cleavage tags pSok-6 for further rapid degradation by PNPase from its 3' end. We also show that Sok RNA is polyadenylated by poly(A) polymerase I (PAP I), and that the poly(A)-tailing is prerequisite for the rapid 3'-exonucleolytic degradation by PNPase.

When rDNA transcription is arrested during mitosis, UBF is still associated with non-condensed rDNA

The mechanisms that control inactivation of ribosomal gene (rDNA) transcription during mitosis is still an open question. To investigate this fundamental question, the precise timing of mitotic arrest was established. In PtK1 cells, rDNA transcription was still active in prophase, stopped in prometaphase until early anaphase, and activated in late anaphase. Because rDNA transcription can still occur in prophase and late anaphase chromosomes, the kinetics of rDNA condensation during mitosis was questioned. The conformation of the rDNA was analyzed by electron microscopy from the G2/M transition to late anaphase in the secondary constriction, the chromosome regions where the rDNAs are clustered. Whether at transcribing or non-transcribing stages, non-condensed rDNA was observed in addition to axial condensed rDNA. Thus, the persistence of this non-condensed rDNA during inactive transcription argues in favor of the fact that mitotic inactivation is not the consequence of rDNA condensation. Analysis of the three-dimensional distribution of the rDNA transcription factor, UBF, revealed that it was similar at each stage of mitosis in the secondary constriction. In addition, the colocalization of UBF with non-condensed rDNA was demonstrated. This is the first visual evidence of the association of UBF with non-condensed rDNA. As we previously reported that the rDNA transcription machinery remained assembled during mitosis, the colocalization of rDNA fibers with UBF argues in favor of the association of the transcription machinery with certain rDNA copies even in the absence of transcription. If this hypothesis is correct, it can be assumed that condensation of rDNA as well as dissociation of the transcription machinery from rDNA cannot explain the arrest of rDNA transcription during mitosis. It is proposed that modifications of the transcription machinery occurring in prometaphase could explain the arrest of transcription, while reverse modifications in late anaphase could explain activation.