Silencing in yeast rDNA chromatin: reciprocal relationship in gene expression between RNA polymerase I and II

rDNA transcription, replication and stability in Saccharomyces cerevisiae

The ribosomal DNA locus (rDNA) is central for the functioning of cells because it encodes ribosomal RNAs, key components of ribosomes, and also because of its links to fundamental metabolic processes, with significant impact on genome integrity and aging. The repetitive nature of the rDNA gene units forces the locus to maintain sequence homogeneity through recombination processes that are closely related to genomic stability. The co-presence of basic DNA transactions, such as replication, transcription by major RNA polymerases, and recombination, in a defined and restricted area of the genome is of particular relevance as it affects the stability of the rDNA locus by both direct and indirect mechanisms. This condition is well exemplified by the rDNA of Saccharomyces cerevisiae. In this review we summarize essential knowledge on how the complexity and overlap of different processes contribute to the control of rDNA and genomic stability in this model organism.

Shaping the chromatin landscape at rRNA and tRNA genes, an emerging new role for RNA polymerase II transcription?

Eukaryotic genes must be condensed into chromatin while remaining accessible to the transcriptional machinery to support gene expression. Among the three eukaryotic RNA polymerases (RNAP), RNAPII is unique, partly because of the C-terminal domain (CTD) of its largest subunit, Rpb1. Rpb1 CTD can be extensively modified during the transcription cycle, allowing for the co-transcriptional recruitment of specific interacting proteins. These include chromatin remodeling factors that control the opening or closing of chromatin. How the CTD-less RNAPI and RNAPIII deal with chromatin at rRNA and tRNA genes is less understood. Here, we review recent advances in our understanding of how the chromatin at tRNA genes and rRNA genes can be remodeled in response to environmental cues in yeast, with a particular focus on the role of local RNAPII transcription in recruiting chromatin remodelers at these loci. In fission yeast, RNAPII transcription at tRNA genes is important to re-establish a chromatin environment permissive to tRNA transcription, which supports growth from stationary phase. In contrast, local RNAPII transcription at rRNA genes correlates with the closing of the chromatin in starvation in budding and fission yeast, suggesting a role in establishing silent chromatin. These opposite roles might support a general model where RNAPII transcription recruits chromatin remodelers to tRNA and rRNA genes to promote the closing and reopening of chromatin in response to the environment.

Ubiquitin proteomics identifies RNA polymerase I as a target of the Smc5/6 complex

Ubiquitination controls numerous cellular processes, and its deregulation is associated with many pathologies. The Nse1 subunit in the Smc5/6 complex contains a RING domain with ubiquitin E3 ligase activity and essential functions in genome integrity. However, Nse1-dependent ubiquitin targets remain elusive. Here, we use label-free quantitative proteomics to analyze the nuclear ubiquitinome of nse1-C274A RING mutant cells. Our results show that Nse1 impacts the ubiquitination of several proteins involved in ribosome biogenesis and metabolism that, importantly, extend beyond canonical functions of Smc5/6. In addition, our analysis suggests a connection between Nse1 and RNA polymerase I (RNA Pol I) ubiquitination. Specifically, Nse1 and the Smc5/6 complex promote ubiquitination of K408 and K410 in the clamp domain of Rpa190, a modification that induces its degradation in response to blocks in transcriptional elongation. We propose that this mechanism contributes to Smc5/6-dependent segregation of the rDNA array, the locus transcribed by RNA Pol I.

Transcription by the Three RNA Polymerases under the Control of the TOR Signaling Pathway in Saccharomyces cerevisiae

Ribosomes are the basis for protein production, whose biogenesis is essential for cells to drive growth and proliferation. Ribosome biogenesis is highly regulated in accordance with cellular energy status and stress signals. In eukaryotic cells, response to stress signals and the production of newly-synthesized ribosomes require elements to be transcribed by the three RNA polymerases (RNA pols). Thus, cells need the tight coordination of RNA pols to adjust adequate components production for ribosome biogenesis which depends on environmental cues. This complex coordination probably occurs through a signaling pathway that links nutrient availability with transcription. Several pieces of evidence strongly support that the Target of Rapamycin (TOR) pathway, conserved among eukaryotes, influences the transcription of RNA pols through different mechanisms to ensure proper ribosome components production. This review summarizes the connection between TOR and regulatory elements for the transcription of each RNA pol in the budding yeast Saccharomyces cerevisiae. It also focuses on how TOR regulates transcription depending on external cues. Finally, it discusses the simultaneous coordination of the three RNA pols through common factors regulated by TOR and summarizes the most important similarities and differences between S. cerevisiae and mammals.

Commuting to Work: Nucleolar Long Non-Coding RNA Control Ribosome Biogenesis from Near and Far

Gene expression is an essential process for cellular growth, proliferation, and differentiation. The transcription of protein-coding genes and non-coding loci depends on RNA polymerases. Interestingly, numerous loci encode long non-coding (lnc)RNA transcripts that are transcribed by RNA polymerase II (RNAPII) and fine-tune the RNA metabolism. The nucleolus is a prime example of how different lncRNA species concomitantly regulate gene expression by facilitating the production and processing of ribosomal (r)RNA for ribosome biogenesis. Here, we summarise the current findings on how RNAPII influences nucleolar structure and function. We describe how RNAPII-dependent lncRNA can both promote nucleolar integrity and inhibit ribosomal (r)RNA synthesis by modulating the availability of rRNA synthesis factors in trans. Surprisingly, some lncRNA transcripts can directly originate from nucleolar loci and function in cis. The nucleolar intergenic spacer (IGS), for example, encodes nucleolar transcripts that counteract spurious rRNA synthesis in unperturbed cells. In response to DNA damage, RNAPII-dependent lncRNA originates directly at broken ribosomal (r)DNA loci and is processed into small ncRNA, possibly to modulate DNA repair. Thus, lncRNA-mediated regulation of nucleolar biology occurs by several modes of action and is more direct than anticipated, pointing to an intimate crosstalk of RNA metabolic events.

Non-Coding, RNAPII-Dependent Transcription at the Promoters of rRNA Genes Regulates Their Chromatin State in S. cerevisiae

Pervasive transcription is widespread in eukaryotes, generating large families of non-coding RNAs. Such pervasive transcription is a key player in the regulatory pathways controlling chromatin state and gene expression. Here, we describe long non-coding RNAs generated from the ribosomal RNA gene promoter called UPStream-initiating transcripts (UPS). In yeast, rDNA genes are organized in tandem repeats in at least two different chromatin states, either transcribed and largely depleted of nucleosomes (open) or assembled in regular arrays of nucleosomes (closed). The production of UPS transcripts by RNA Polymerase II from endogenous rDNA genes was initially documented in mutants defective for rRNA production by RNA polymerase I. We show here that UPS are produced in wild-type cells from closed rDNA genes but are hidden within the enormous production of rRNA. UPS levels are increased when rDNA chromatin states are modified at high temperatures or entering/leaving quiescence. We discuss their role in the regulation of rDNA chromatin states and rRNA production.

Exonuclease resistant 18S and 25S ribosomal RNA components in yeast are possibly newly transcribed by RNA polymerase II

Background

We have previously reported 18S and 25S ribosomal RNA molecules in Candida albicans resistant to processive 5′ → 3′ exonuclease, appearing as cells approached stationary growth phase. Initial analysis pointed to extra phosphate(s) at their 5′- end raising the possibility that they were newly transcribed. Here we report on additional experiments exploring this possibility and try to establish which of the RNA polymerases may be transcribing them.

Results

Oligo-ligation and primer extension again showed the presence of extra phosphate at the 5′-end of the reported processing sites for both 18S and 25S ribosomal RNA components. Inhibition of Pol I with BMH-21 increased the presence of the molecules. Quantitation with an Agilent Bioanalyzer showed that resistant 18S and 25S molecules are primarily produced in the nucleus. Utilizing an RNA cap specific antibody, a signal could be detected on these molecules via immunoblotting; such signal could be eliminated by decapping reaction. Both the cap specific antibody and eIF4E cap-binding protein, increased fold enrichment upon quantitative amplification. Antibodies specific for the RNA Polymerase II c-terminal domain and TFIIB initiator factor showed the presence of Pol II on DNA sequences for both 18S and 25S molecules in chromatin precipitation and qPCR assays. Rapamycin inhibition of TOR complex also resulted in an increase of resistant 18S and 25S molecules.

Conclusions

These data raise the possibility of a role for RNA Polymerase II in the production of 18S and 25S molecules and indicate that efforts for more direct proof may be worthwhile. If definitively proven it will establish an additional role for RNA Polymerase II in ribosomal production.

Nascent Transcript Folding Plays a Major Role in Determining RNA Polymerase Elongation Rates

Transcription elongation rates influence RNA processing, but sequence-specific regulation is poorly understood. We addressed this in vivo, analyzing RNAPI in S. cerevisiae. Mapping RNAPI by Miller chromatin spreads or UV crosslinking revealed 5' enrichment and strikingly uneven local polymerase occupancy along the rDNA, indicating substantial variation in transcription speed. Two features of the nascent transcript correlated with RNAPI distribution: folding energy and GC content in the transcription bubble. In vitro experiments confirmed that strong RNA structures close to the polymerase promote forward translocation and limit backtracking, whereas high GC in the transcription bubble slows elongation. A mathematical model for RNAPI elongation confirmed the importance of nascent RNA folding in transcription. RNAPI from S. pombe was similarly sensitive to transcript folding, as were S. cerevisiae RNAPII and RNAPIII. For RNAPII, unstructured RNA, which favors slowed elongation, was associated with faster cotranscriptional splicing and proximal splice site use, indicating regulatory significance for transcript folding.

Non-coding RNAs at the Eukaryotic rDNA Locus: RNA-DNA Hybrids and Beyond

The human ribosomal DNA (rDNA) locus encodes a variety of long non-coding RNAs (lncRNAs). Among them, the canonical ribosomal RNAs that are the catalytic components of the ribosomes, as well as regulatory lncRNAs including promoter-associated RNAs (pRNA), stress-induced promoter and pre-rRNA antisense RNAs (PAPAS), and different intergenic spacer derived lncRNA species (IGSRNA). In addition, externally encoded lncRNAs are imported into the nucleolus, which orchestrate the complex regulation of the nucleolar state in normal and stress conditions via a plethora of molecular mechanisms. This review focuses on the triplex and R-loop formation aspects of lncRNAs at the rDNA locus in yeast and human cells. We discuss the protein players that regulate R-loops at rDNA and how their misregulation contributes to DNA damage and disease. Furthermore, we speculate how DNA lesions such as rNMPs or 8-oxo-dG might affect RNA-DNA hybrid formation. The transcription of lncRNA from rDNA has been observed in yeast, plants, flies, worms, mouse and human cells. This evolutionary conservation highlights the importance of lncRNAs in rDNA function and maintenance.

Heterologous coexpression of the benzoate-para-hydroxylase CYP53B1 with different cytochrome P450 reductases in various yeasts

Cytochrome P450 monooxygenases (P450) are enzymes with high potential as biocatalysts for industrial applications. Their large-scale applications are, however, limited by instability and requirement for coproteins and/or expensive cofactors. These problems are largely overcome when whole cells are used as biocatalysts. We previously screened various yeast species heterologously expressing self-sufficient P450s for their potential as whole-cell biocatalysts. Most P450s are, however, not self-sufficient and consist of two or three protein component systems. Therefore, in the present study, we screened different yeast species for coexpression of P450 and P450-reductase (CPR) partners, using CYP53B1 from Rhodotorula minuta as an exemplary P450. The abilities of three different coexpressed CPR partners to support P450 activity were investigated, two from basidiomycetous origin and one from an ascomycete. The various P450-CPR combinations were cloned into strains of Saccharomyces cerevisiae, Kluyveromyces marxianus, Hansenula polymorpha, Yarrowia lipolytica and Arxula adeninivorans, using a broad-range yeast expression vector. The results obtained supported the previous finding that recombinant A. adeninivorans strains perform excellently as whole-cell biocatalysts. This study also demonstrated for the first time the P450 reductase activity of the CPRs from R. minuta and U. maydis. A very interesting observation was the variation in the supportive activity provided by the different reductase partners tested and demonstrated better P450 activity enhancement by a heterologous CPR compared to its natural partner CPR. This study highlights reductase selection as a critical variable for consideration in the pursuit of optimal P450-based catalytic systems. The usefulness of A. adeninivorans as both a host for recombinant P450s and whole-cell biocatalyst was emphasized, supporting earlier findings.

Endogenous single-strand DNA breaks at RNA polymerase II promoters in Saccharomyces cerevisiae

Molecular combing and gel electrophoretic studies revealed endogenous nicks with free 3'OH ends at ∼100 kb intervals in the genomic DNA (gDNA) of unperturbed and G1-synchronized Saccharomyces cerevisiae cells. Analysis of the distribution of endogenous nicks by Nick ChIP-chip indicated that these breaks accumulated at active RNA polymerase II (RNAP II) promoters, reminiscent of the promoter-proximal transient DNA breaks of higher eukaryotes. Similar periodicity of endogenous nicks was found within the ribosomal rDNA cluster, involving every ∼10th of the tandemly repeated 9.1 kb units of identical sequence. Nicks were mapped by Southern blotting to a few narrow regions within the affected units. Three of them were overlapping the RNAP II promoters, while the ARS-containing IGS2 region was spared of nicks. By using a highly sensitive reverse-Southwestern blot method to map free DNA ends with 3'OH, nicks were shown to be distinct from other known rDNA breaks and linked to the regulation of rDNA silencing. Nicks in rDNA and the rest of the genome were typically found at the ends of combed DNA molecules, occasionally together with R-loops, comprising a major pool of vulnerable sites that are connected with transcriptional regulation.

Genetic analyses led to the discovery of a super-active mutant of the RNA polymerase I

Most transcriptional activity of exponentially growing cells is carried out by the RNA Polymerase I (Pol I), which produces a ribosomal RNA (rRNA) precursor. In budding yeast, Pol I is a multimeric enzyme with 14 subunits. Among them, Rpa49 forms with Rpa34 a Pol I-specific heterodimer (homologous to PAF53/CAST heterodimer in human Pol I), which might be responsible for the specific functions of the Pol I. Previous studies provided insight in the involvement of Rpa49 in initiation, elongation, docking and releasing of Rrn3, an essential Pol I transcription factor. Here, we took advantage of the spontaneous occurrence of extragenic suppressors of the growth defect of the rpa49 null mutant to better understand the activity of Pol I. Combining genetic approaches, biochemical analysis of rRNA synthesis and investigation of the transcription rate at the individual gene scale, we characterized mutated residues of the Pol I as novel extragenic suppressors of the growth defect caused by the absence of Rpa49. When mapped on the Pol I structure, most of these mutations cluster within the jaw-lobe module, at an interface formed by the lobe in Rpa135 and the jaw made up of regions of Rpa190 and Rpa12. In vivo, the suppressor allele RPA135-F301S restores normal rRNA synthesis and increases Pol I density on rDNA genes when Rpa49 is absent. Growth of the Rpa135-F301S mutant is impaired when combined with exosome mutation rrp6Δ and it massively accumulates pre-rRNA. Moreover, Pol I bearing Rpa135-F301S is a hyper-active RNA polymerase in an in vitro tailed-template assay. We conclude that RNA polymerase I can be engineered to produce more rRNA in vivo and in vitro. We propose that the mutated area undergoes a conformational change that supports the DNA insertion into the cleft of the enzyme resulting in a super-active form of Pol I.

The nuclear genome of eukaryotic cells is transcribed by three RNA polymerases. RNA polymerase I (Pol I) is a multimeric enzyme specialized in the synthesis of ribosomal RNA. Deregulation of the Pol I function is linked to the etiology of a broad range of human diseases. Understanding the Pol I activity and regulation represents therefore a major challenge. We chose the budding yeast Saccharomyces cerevisiae as a model, because Pol I transcription apparatus is genetically amenable in this organism. Analyses of phenotypic consequences of deletion/truncation of Pol I subunits-coding genes in yeast indeed provided insights into the activity and regulation of the enzyme. Here, we characterized mutations in Pol I that can alleviate the growth defect caused by the absence of Rpa49, one of the subunits composing this multi-protein enzyme. We mapped these mutations on the Pol I structure and found that they all cluster in a well-described structural element, the jaw-lobe module. Combining genetic and biochemical approaches, we showed that Pol I bearing one of these mutations in the Rpa135 subunit is able to produce more ribosomal RNA in vivo and in vitro. We propose that this super-activity is explained by structural rearrangement of the Pol I jaw/lobe interface.

Fob1p recruits DNA topoisomerase I to ribosomal genes locus and contributes to its transcriptional silencing maintenance

S. cerevisiae ribosomal DNA (rDNA) locus hosts a series of highly complex regulatory machineries for RNA polymerase I, II and III transcription, DNA replication and units recombination, all acting in the Non Transcribed Spacers (NTSs) interposed between the repeated units by which it is composed. DNA topoisomerase I (Top1p) contributes, recruiting Sir2p, to the maintenance of transcriptional silencing occurring at the RNA Polymerase II cryptic promoters, located in the NTS region. In this paper we found that Fob1p presence is crucial for Top1p recruitment at NTS, allowing transcriptional silencing to be established and maintained. We also showed the role of Nsr1p in Top1p recruitment to rDNA locus. Our work allows to hypothesize that Nsr1p targets Top1p into the nucleolus while Fob1p is responsible for its preferential distribution at RFB.

Condensin ATPase motifs contribute differentially to the maintenance of chromosome morphology and genome stability

Effective transfer of genetic information during cell division requires a major reorganization of chromosome structure. This process is triggered by condensin, a conserved pentameric ATPase essential for chromosome condensation. How condensin harnesses the energy of ATP hydrolysis to promote chromatin reorganization is unknown. To address this issue, we performed a genetic screen specifically focused on the ATPase domain of Smc4, a core subunit of condensin. Our screen identified mutational hotspots that impair condensin’s ability to condense chromosomes to various degrees. These mutations have distinct effects on viability, genome stability, and chromosome morphology, revealing unique thresholds for condensin enzymatic activity in the execution of its cellular functions. Biochemical analyses indicate that inactivation of Smc4 ATPase activity can result in cell lethality because it favors a specific configuration of condensin that locks ATP in the enzyme. Together, our results provide critical insights into the mechanism used by condensin to harness the energy of ATP hydrolysis for the compaction of chromatin.

In eukaryotes, the deletion of a single copy of most genes shows little or no detectable phenotype under standard proliferative conditions. This implies that a large reduction in the level of a gene product can be tolerated by eukaryotic organisms and that a “reserve capacity” is built in the protein machinery that drives most cellular processes. Here, we test if the main effector of chromosome condensation—the condensin complex—operates with a reserve enzymatic capacity in the execution of its multiple functions in vivo. To achieve this, we created an allelic series of mutations that selectively inactivate condensin ATPase activity in a graded manner. We show that many core functions of condensin can be maintained even at low levels of ATPase activity. Our data also reveal the existence of various thresholds of ATPase activity that are necessary and sufficient for the execution of different cellular functions by condensin. Notably, loss of genome stability at repetitive DNA is only observed when condensin ATPase activity is severely impaired. Taken together, our results reveal key insights into the process of ATP hydrolysis by condensin and how the energy it releases promotes genome remodeling and stability during cell division.

The Nuts and Bolts of Transcriptionally Silent Chromatin in Saccharomyces cerevisiae

Transcriptional silencing in Saccharomyces cerevisiae occurs at several genomic sites including the silent mating-type loci, telomeres, and the ribosomal DNA (rDNA) tandem array. Epigenetic silencing at each of these domains is characterized by the absence of nearly all histone modifications, including most prominently the lack of histone H4 lysine 16 acetylation. In all cases, silencing requires Sir2, a highly-conserved NAD(+)-dependent histone deacetylase. At locations other than the rDNA, silencing also requires additional Sir proteins, Sir1, Sir3, and Sir4 that together form a repressive heterochromatin-like structure termed silent chromatin. The mechanisms of silent chromatin establishment, maintenance, and inheritance have been investigated extensively over the last 25 years, and these studies have revealed numerous paradigms for transcriptional repression, chromatin organization, and epigenetic gene regulation. Studies of Sir2-dependent silencing at the rDNA have also contributed to understanding the mechanisms for maintaining the stability of repetitive DNA and regulating replicative cell aging. The goal of this comprehensive review is to distill a wide array of biochemical, molecular genetic, cell biological, and genomics studies down to the "nuts and bolts" of silent chromatin and the processes that yield transcriptional silencing.

RNA Polymerase I and Fob1 contributions to transcriptional silencing at the yeast rDNA locus

RNA polymerase II (Pol II)-transcribed genes embedded within the yeast rDNA locus are repressed through a Sir2-dependent process called ‘rDNA silencing’. Sir2 is recruited to the rDNA promoter through interactions with RNA polymerase I (Pol I), and to a pair of DNA replication fork block sites (Ter1 and Ter2) through interaction with Fob1. We utilized a reporter gene (mURA3) integrated adjacent to the leftmost rDNA gene to investigate localized Pol I and Fob1 functions in silencing. Silencing was attenuated by loss of Pol I subunits or insertion of an ectopic Pol I terminator within the adjacent rDNA gene. Silencing left of the rDNA array is naturally attenuated by the presence of only one intact Fob1 binding site (Ter2). Repair of the 2nd Fob1 binding site (Ter1) dramatically strengthens silencing such that it is no longer impacted by local Pol I transcription defects. Global loss of Pol I activity, however, negatively affects Fob1 association with the rDNA. Loss of Ter2 almost completely eliminates localized silencing, but is restored by artificially targeting Fob1 or Sir2 as Gal4 DNA binding domain fusions. We conclude that Fob1 and Pol I make independent contributions to establishment of silencing, though Pol I also reinforces Fob1-dependent silencing.

The Transcription Factor THO Promotes Transcription Initiation and Elongation by RNA Polymerase I

Although ribosomal RNA represents the majority of cellular RNA, and ribosome synthesis is closely connected to cell growth and proliferation rates, a complete understanding of the factors that influence transcription of ribosomal DNA is lacking. Here, we show that the THO complex positively affects transcription by RNA polymerase I (Pol I). We found that THO physically associates with the rDNA repeat and interacts genetically with Pol I transcription initiation factors. Pol I transcription in hpr1 or tho2 null mutants is dramatically reduced to less than 20% of the WT level. Pol I occupancy of the coding region of the rDNA in THO mutants is decreased to ~50% of WT level. Furthermore, although the percentage of active rDNA repeats remains unaffected in the mutant cells, the overall rDNA copy number increases ~2-fold compared with WT. Together, these data show that perturbation of THO function impairs transcription initiation and elongation by Pol I, identifying a new cellular target for the conserved THO complex.

Regulation of ribosomal DNA amplification by the TOR pathway

Repeated regions are widespread in eukaryotic genomes, and key functional elements such as the ribosomal DNA tend to be formed of high copy repeated sequences organized in tandem arrays. In general, high copy repeats are remarkably stable, but a number of organisms display rapid ribosomal DNA amplification at specific times or under specific conditions. Here we demonstrate that target of rapamycin (TOR) signaling stimulates ribosomal DNA amplification in budding yeast, linking external nutrient availability to ribosomal DNA copy number. We show that ribosomal DNA amplification is regulated by three histone deacetylases: Sir2, Hst3, and Hst4. These enzymes control homologous recombination-dependent and nonhomologous recombination-dependent amplification pathways that act in concert to mediate rapid, directional ribosomal DNA copy number change. Amplification is completely repressed by rapamycin, an inhibitor of the nutrient-responsive TOR pathway; this effect is separable from growth rate and is mediated directly through Sir2, Hst3, and Hst4. Caloric restriction is known to up-regulate expression of nicotinamidase Pnc1, an enzyme that enhances Sir2, Hst3, and Hst4 activity. In contrast, normal glucose concentrations stretch the ribosome synthesis capacity of cells with low ribosomal DNA copy number, and we find that these cells show a previously unrecognized transcriptional response to caloric excess by reducing PNC1 expression. PNC1 down-regulation forms a key element in the control of ribosomal DNA amplification as overexpression of PNC1 substantially reduces ribosomal DNA amplification rate. Our results reveal how a signaling pathway can orchestrate specific genome changes and demonstrate that the copy number of repetitive DNA can be altered to suit environmental conditions.

The nucleolus: a raft adrift in the nuclear sea or the keystone in nuclear structure?

The nucleolus is a prominent nuclear structure that is the site of ribosomal RNA (rRNA) transcription, and hence ribosome biogenesis. Cellular demand for ribosomes, and hence rRNA, is tightly linked to cell growth and the rRNA makes up the majority of all the RNA within a cell. To fulfill the cellular demand for rRNA, the ribosomal RNA (rDNA) genes are amplified to high copy number and transcribed at very high rates. As such, understanding the rDNA has profound consequences for our comprehension of genome and transcriptional organization in cells. In this review, we address the question of whether the nucleolus is a raft adrift the sea of nuclear DNA, or actively contributes to genome organization. We present evidence supporting the idea that the nucleolus, and the rDNA contained therein, play more roles in the biology of the cell than simply ribosome biogenesis. We propose that the nucleolus and the rDNA are central factors in the spatial organization of the genome, and that rapid alterations in nucleolar structure in response to changing conditions manifest themselves in altered genomic structures that have functional consequences. Finally, we discuss some predictions that result from the nucleolus having a central role in nuclear organization.

Yeast sirtuins and the regulation of aging

The sirtuins are a phylogenetically conserved family of NAD(+) -dependent protein deacetylases that consume one molecule of NAD(+) for every deacetylated lysine side chain. Their requirement for NAD(+) potentially makes them prone to regulation by fluctuations in NAD(+) or biosynthesis intermediates, thus linking them to cellular metabolism. The Sir2 protein from Saccharomyces cerevisiae is the founding sirtuin family member and has been well characterized as a histone deacetylase that functions in transcriptional silencing of heterochromatin domains and as a pro-longevity factor for replicative life span (RLS), defined as the number of times a mother cell divides (buds) before senescing. Deleting SIR2 shortens RLS, while increased gene dosage causes extension. Furthermore, Sir2 has been implicated in mediating the beneficial effects of caloric restriction (CR) on life span, not only in yeast, but also in higher eukaryotes. While this paradigm has had its share of disagreements and debate, it has also helped rapidly drive the aging research field forward. S. cerevisiae has four additional sirtuins, Hst1, Hst2, Hst3, and Hst4. This review discusses the function of Sir2 and the Hst homologs in replicative aging and chronological aging, and also addresses how the sirtuins are regulated in response to environmental stresses such as CR.

A user's guide to the ribosomal DNA in Saccharomyces cerevisiae

Messenger RNA synthesis (mRNA) accounts for a small fraction of total RNA synthesis in growing eukaryotic cells. The bulk of cellular transcription is devoted to ribosomal RNA (rRNA) synthesis (Warner, Trends Biochem Sci 24:437-440, 1999). Several unique characteristics of the rDNA and RNA polymerase I must be considered in order to accurately quantify the synthesis rate of rRNA or to characterize its processing. Indeed, an entirely different set of techniques must be applied to the study of rRNA synthesis than is routinely to study mRNA synthesis. Five of the most useful strategies for genetic and molecular analysis of rRNA synthesis and regulation are outlined in this chapter. The techniques described were developed for characterization of the model eukaryote Saccharomyces cerevisiae; however, many of these strategies can be adapted for studies in other eukaryotic cells.

Subnuclear relocalization and silencing of a chromosomal region by an ectopic ribosomal DNA repeat

Our research addresses the relationship between subnuclear localization and gene expression in fission yeast. We observed the relocalization of a heterochromatic region, the mating-type region, from its natural location at the spindle-pole body to the immediate vicinity of the nucleolus. Relocalization occurred in response to a DNA rearrangement replacing a boundary element (IR-R) with a ribosomal DNA repeat (rDNA-R). Gene expression was strongly silenced in the relocalized mating-type region through mechanisms that differ from those operating in wild type. Also different from the wild-type situation, programmed recombination events failed to take place in the rDNA-R mutant. Increased silencing and perinucleolar localization depended on Reb1, a DNA-binding protein with cognate sites in the rDNA. Reb1 was recently shown to mediate long-range interchromosomal interactions in the nucleus through dimerization, providing a mechanism for the observed relocalization. Replacing the full rDNA repeat with Reb1-binding sites, and using mutants lacking the histone H3K9 methyltransferase Clr4, indicated that the relocalized region was silenced redundantly by heterochromatin and another mechanism, plausibly antisense transcription, achieving a high degree of repression in the rDNA-R strain.

Compositional and structural analysis of selected chromosomal domains from Saccharomyces cerevisiae

Chromatin is the template for replication and transcription in the eukaryotic nucleus, which needs to be defined in composition and structure before these processes can be fully understood. We report an isolation protocol for the targeted purification of specific genomic regions in their native chromatin context from Saccharomyces cerevisiae. Subdomains of the multicopy ribosomal DNA locus containing transcription units of RNA polymerases I, II or III or an autonomous replication sequence were independently purified in sufficient amounts and purity to analyze protein composition and histone modifications by mass spectrometry. We present and discuss the proteomic data sets obtained for chromatin in different functional states. The native chromatin was further amenable to electron microscopy analysis yielding information about nucleosome occupancy and positioning at the single-molecule level. We also provide evidence that chromatin from virtually every single copy genomic locus of interest can be purified and analyzed by this technique.

Structure-function analysis of Hmo1 unveils an ancestral organization of HMG-Box factors involved in ribosomal DNA transcription from yeast to human

Ribosome biogenesis is a major metabolic effort for growing cells. In Saccharomyces cerevisiae, Hmo1, an abundant high-mobility group box protein (HMGB) binds to the coding region of the RNA polymerase I transcribed ribosomal RNAs genes and the promoters of ∼70% of ribosomal protein genes. In this study, we have demonstrated the functional conservation of eukaryotic HMGB proteins involved in ribosomal DNA (rDNA) transcription. We have shown that when expressed in budding yeast, human UBF1 and a newly identified Sp-Hmo1 (Schizosaccharomyces pombe) localize to the nucleolus and suppress growth defect of the RNA polymerase I mutant rpa49-Δ. Owing to the multiple functions of both proteins, Hmo1 and UBF1 are not fully interchangeable. By deletion and domains swapping in Hmo1, we identified essential domains that stimulate rDNA transcription but are not fully required for stimulation of ribosomal protein genes expression. Hmo1 is organized in four functional domains: a dimerization module, a canonical HMGB motif followed by a conserved domain and a C-terminal nucleolar localization signal. We propose that Hmo1 has acquired species-specific functions and shares with UBF1 and Sp-Hmo1 an ancestral function to stimulate rDNA transcription.

Divergent contributions of conserved active site residues to transcription by eukaryotic RNA polymerases I and II

Multisubunit RNA polymerases (msRNAPs) exhibit high sequence and structural homology, especially within their active sites, which is generally thought to result in msRNAP functional conservation. However, we show that mutations in the trigger loop (TL) in the largest subunit of RNA polymerase I (Pol I) yield phenotypes unexpected from studies of Pol II. For example, a well-characterized gain-of-function mutation in Pol II results in loss of function in Pol I (Pol II: rpb1- E1103G; Pol I: rpa190-E1224G). Studies of chimeric Pol II enzymes hosting Pol I or Pol III TLs suggest that consequences of mutations that alter TL dynamics are dictated by the greater enzymatic context and not solely the TL sequence. Although the rpa190-E1224G mutation diminishes polymerase activity, when combined with mutations that perturb Pol I catalysis, it enhances polymerase function, similar to the analogous Pol II mutation. These results suggest that Pol I and Pol II have different rate-limiting steps.

Spatial telomere organization and clustering in yeast Saccharomyces cerevisiae nucleus is generated by a random dynamics of aggregation-dissociation

The 32 telomeres of budding yeast form clusters, yet whether clusters are due to random localization or telomeric interactions is unclear. Data from live-cell imaging are compared with a biophysical model of telomere dynamics. Direct molecular interaction between telomeres is the key parameter that regulates telomere clustering.

Spatial and temporal behavior of chromosomes and their regulatory proteins is a key control mechanism in genomic function. This is exemplified by the clustering of the 32 budding yeast telomeres that form foci in which silencing factors concentrate. To uncover the determinants of telomere distribution, we compare live-cell imaging with a stochastic model of telomere dynamics that we developed. We show that random encounters alone are inadequate to produce the clustering observed in vivo. In contrast, telomere dynamics observed in vivo in both haploid and diploid cells follows a process of dissociation–aggregation. We determine the time that two telomeres spend in the same cluster for the telomere distribution observed in cells expressing different levels of the silencing factor Sir3 protein, limiting for telomere clustering. We conclude that telomere clusters, their dynamics, and their nuclear distribution result from random motion, aggregation, and dissociation of telomeric regions, specifically determined by the amount of Sir3.

The SWI/SNF chromatin remodeling complex influences transcription by RNA polymerase I in Saccharomyces cerevisiae

SWI/SNF is a chromatin remodeling complex that affects transcription initiation and elongation by RNA polymerase II. Here we report that SWI/SNF also plays a role in transcription by RNA polymerase I (Pol I) in Saccharomyces cerevisiae. Deletion of the genes encoding the Snf6p or Snf5p subunits of SWI/SNF was lethal in combination with mutations that impair Pol I transcription initiation and elongation. SWI/SNF physically associated with ribosomal DNA (rDNA) within the coding region, with an apparent peak near the 5' end of the gene. In snf6Δ cells there was a ∼2.5-fold reduction in rRNA synthesis rate compared to WT, but there was no change in average polymerase occupancy per gene, the number of rDNA gene repeats, or the percentage of transcriptionally active rDNA genes. However, both ChIP and EM analyses showed a small but reproducible increase in Pol I density in a region near the 5' end of the gene. Based on these data, we conclude that SWI/SNF plays a positive role in Pol I transcription, potentially by modifying chromatin structure in the rDNA repeats. Our findings demonstrate that SWI/SNF influences the most robust transcription machinery in proliferating cells.

RNAP-II molecules participate in the anchoring of the ORC to rDNA replication origins

The replication of genomic DNA is limited to a single round per cell cycle. The first component, which recognises and remains bound to origins from recognition until activation and replication elongation, is the origin recognition complex. How origin recognition complex (ORC) proteins remain associated with chromatin throughout the cell cycle is not yet completely understood. Several genome-wide studies have undoubtedly demonstrated that RNA polymerase II (RNAP-II) binding sites overlap with replication origins and with the binding sites of the replication components. RNAP-II is no longer merely associated with transcription elongation. Several reports have demonstrated that RNAP-II molecules affect chromatin structure, transcription, mRNA processing, recombination and DNA repair, among others. Most of these activities have been reported to directly depend on the interaction of proteins with the C-terminal domain (CTD) of RNAP-II. Two-dimensional gels results and ChIP analysis presented herein suggest that stalled RNAP-II molecules bound to the rDNA chromatin participate in the anchoring of ORC proteins to origins during the G1 and S-phases. The results show that in the absence of RNAP-II, Orc1p, Orc2p and Cdc6p do not bind to origins. Moreover, co-immunoprecipitation experiments suggest that Ser2P-CTD and hypophosphorylated RNAP-II interact with Orc1p. In the context of rDNA, cryptic transcription by RNAP-II did not negatively interfere with DNA replication. However, the results indicate that RNAP-II is not necessary to maintain the binding of ORCs to the origins during metaphase. These findings highlight for the first time the potential importance of stalled RNAP-II in the regulation of DNA replication.

RNA polymerase I termination: Where is the end?

The synthesis of ribosomal RNA (rRNA) precursor molecules by RNA polymerase I (Pol I) terminates with the dissociation of the protein-DNA-RNA ternary complex. Based on in vitro results the mechanism of Pol I termination appeared initially to be rather conserved and simple until this process was more thoroughly re-investigated in vivo. A picture emerged that Pol I termination seems to be connected to co-transcriptional processing, re-initiation of transcription and, possibly, other processes downstream of Pol I transcription units. In this article, our current understanding of the mechanism of Pol I termination and how this process might be implicated in other biological processes in yeast and mammals is summarized and discussed. This article is part of a Special Issue entitled: Transcription by Odd Pols.

Targeting of the human coagulation factor IX gene at rDNA locus of human embryonic stem cells

BACKGROUND

Genetic modification is a prerequisite to realizing the full potential of human embryonic stem cells (hESCs) in human genetic research and regenerative medicine. Unfortunately, the random integration methods that have been the primary techniques used keep creating problems, and the primary alternative method, gene targeting, has been effective in manipulating mouse embryonic stem cells (mESCs) but poorly in hESCs.

METHODOLOGY/PRINCIPAL FINDINGS

Human ribosomal DNA (rDNA) repeats are clustered on the short arm of acrocentric chromosomes. They consist of approximately 400 copies of the 45S pre-RNA (rRNA) gene per haploid. In the present study, we targeted a physiological gene, human coagulation factor IX, into the rDNA locus of hESCs via homologous recombination. The relative gene targeting efficiency (>50%) and homologous recombination frequency (>10(-5)) were more than 10-fold higher than those of loci targeted in previous reports. Meanwhile, the targeted clones retained both a normal karyotype and the main characteristics of ES cells. The transgene was found to be stably and ectopically expressed in targeted hESCs.

CONCLUSION/SIGNIFICANCE

This is the first targeting of a human physiological gene at a defined locus on the hESC genome. Our findings indicate that the rDNA locus may serve as an ideal harbor for transgenes in hESCs.

RNase H and multiple RNA biogenesis factors cooperate to prevent RNA:DNA hybrids from generating genome instability

Genome instability, a hallmark of cancer progression, is thought to arise through DNA double strand breaks (DSBs). Studies in yeast and mammalian cells have shown that DSBs and instability can occur through RNA:DNA hybrids generated by defects in RNA elongation and splicing. We report that in yeast hybrids naturally form at many loci in wild-type cells, likely due to transcriptional errors, but are removed by two evolutionarily conserved RNase H enzymes. Mutants defective in transcriptional repression, RNA export and RNA degradation show increased hybrid formation and associated genome instability. One mutant, sin3Δ, changes the genome profile of hybrids, enhancing formation at ribosomal DNA. Hybrids likely induce damage in G1, S and G2/M as assayed by Rad52 foci. In summary, RNA:DNA hybrids are a potent source for changing genome structure. By preventing their formation and accumulation, multiple RNA biogenesis factors and RNase H act as guardians of the genome.

RNA polymerase I activity is regulated at multiple steps in the transcription cycle: recent insights into factors that influence transcription elongation

Synthesis of the translation apparatus is a central activity in growing and/or proliferating cells. Because of its fundamental importance and direct connection to cell proliferation, ribosome synthesis has been a focus of ongoing research for several decades. As a consequence, much is known about the essential factors involved in this process. Many studies have shown that transcription of the ribosomal DNA by RNA polymerase I is a major target for cellular regulation of ribosome synthesis rates. The initiation of transcription by RNA polymerase I has been implicated as a regulatory target, however, recent studies suggest that the elongation step in transcription is also influenced and regulated by trans-acting factors. This review describes the factors required for rRNA synthesis and focuses on recent works that have begun to identify and characterize factors that influence transcription elongation by RNA polymerase I and its regulation.

Repeat expansion in the budding yeast ribosomal DNA can occur independently of the canonical homologous recombination machinery

Major eukaryotic genomic elements, including the ribosomal DNA (rDNA), are composed of repeated sequences with well-defined copy numbers that must be maintained by regulated recombination. Although mechanisms that instigate rDNA recombination have been identified, none are directional and they therefore cannot explain precise repeat number control. Here, we show that yeast lacking histone chaperone Asf1 undergo reproducible rDNA repeat expansions. These expansions do not require the replication fork blocking protein Fob1 and are therefore independent of known rDNA expansion mechanisms. We propose the existence of a regulated rDNA repeat gain pathway that becomes constitutively active in asf1Δ mutants. Cells lacking ASF1 accumulate rDNA repeats with high fidelity in a processive manner across multiple cell divisions. The mechanism of repeat gain is dependent on highly repetitive sequence but, surprisingly, is independent of the homologous recombination proteins Rad52, Rad51 and Rad59. The expansion mechanism is compromised by mutations that decrease the processivity of DNA replication, which leads to progressive loss of rDNA repeats. Our data suggest that a novel mode of break-induced replication occurs in repetitive DNA that is dependent on high homology but does not require the canonical homologous recombination machinery.

The transcription elongation factor Spt5 influences transcription by RNA polymerase I positively and negatively

Spt5p is a universally conserved transcription factor that plays multiple roles in eukaryotic transcription elongation. Spt5p forms a heterodimer with Spt4p and collaborates with other transcription factors to pause or promote RNA polymerase II transcription elongation. We have shown previously that Spt4p and Spt5p also influence synthesis of ribosomal RNA by RNA polymerase (Pol) I; however, previous studies only characterized defects in Pol I transcription induced by deletion of SPT4. Here we describe two new, partially active mutations in SPT5 and use these mutant strains to characterize the effect of Spt5p on Pol I transcription. Genetic interactions between spt5 and rpa49Δ mutations together with measurements of ribosomal RNA synthesis rates, rDNA copy number, and Pol I occupancy of the rDNA demonstrate that Spt5p plays both positive and negative roles in transcription by Pol I. Electron microscopic analysis of mutant and WT strains confirms these observations and supports the model that Spt4/5 may contribute to pausing of RNA polymerase I early during transcription elongation but promotes transcription elongation downstream of the pause(s). These findings bolster the model that Spt5p and related homologues serve diverse critical roles in the control of transcription.

RNA polymerase I-specific subunits promote polymerase clustering to enhance the rRNA gene transcription cycle

RNA polymerase I (Pol I) produces large ribosomal RNAs (rRNAs). In this study, we show that the Rpa49 and Rpa34 Pol I subunits, which do not have counterparts in Pol II and Pol III complexes, are functionally conserved using heterospecific complementation of the human and Schizosaccharomyces pombe orthologues in Saccharomyces cerevisiae. Deletion of RPA49 leads to the disappearance of nucleolar structure, but nucleolar assembly can be restored by decreasing ribosomal gene copy number from 190 to 25. Statistical analysis of Miller spreads in the absence of Rpa49 demonstrates a fourfold decrease in Pol I loading rate per gene and decreased contact between adjacent Pol I complexes. Therefore, the Rpa34 and Rpa49 Pol I-specific subunits are essential for nucleolar assembly and for the high polymerase loading rate associated with frequent contact between adjacent enzymes. Together our data suggest that localized rRNA production results in spatially constrained rRNA production, which is instrumental for nucleolar assembly.

Alternative chromatin structures of the 35S rRNA genes in Saccharomyces cerevisiae provide a molecular basis for the selective recruitment of RNA polymerases I and II

In all eukaryotes, a specialized enzyme, RNA polymerase I (Pol I), is dedicated to transcribe the 35S rRNA gene from a multicopy gene cluster, the ribosomal DNA (rDNA). In certain Saccharomyces cerevisiae mutants, 35S rRNA genes can be transcribed by RNA polymerase II (Pol II). In these mutants, rDNA silencing of Pol II transcription is impaired. It has been speculated that upstream activating factor (UAF), which binds to a specific DNA element within the Pol I promoter, plays a crucial role in forming chromatin structures responsible for polymerase specificity and silencing at the rDNA locus. We therefore performed an in-depth analysis of chromatin structure and composition in different mutant backgrounds. We demonstrate that chromatin architecture of the entire Pol I-transcribed region is substantially altered in the absence of UAF, allowing RNA polymerases II and III to access DNA elements flanking a Pol promoter-proximal Reb1 binding site. Furthermore, lack of UAF leads to the loss of Sir2 from rDNA, correlating with impaired Pol II silencing. This analysis of rDNA chromatin provides a molecular basis, explaining many phenotypes observed in previous genetic analyses.

RNA polymerase I transcription silences noncoding RNAs at the ribosomal DNA locus in Saccharomyces cerevisiae

In Saccharomyces cerevisiae the repeated units of the ribosomal locus, transcribed by RNA polymerase I (Pol I), are interrupted by nontranscribed spacers (NTSs). These NTS regions are transcribed by RNA polymerase III to synthesize 5S RNA and by RNA polymerase II (Pol II) to synthesize, at low levels, noncoding RNAs (ncRNAs). While transcription of both RNA polymerase I and III is highly characterized, at the ribosomal DNA (rDNA) locus only a few studies have been performed on Pol II, whose repression correlates with the SIR2-dependent silencing. The involvement of both chromatin organization and Pol I transcription has been proposed, and peculiar chromatin structures might justify "ribosomal" Pol II silencing. Reporter genes inserted within the rDNA units have been employed for these studies. We studied, in the natural context, yeast mutants differing in Pol I transcription in order to find whether correlations exist between Pol I transcription and Pol II ncRNA production. Here, we demonstrate that silencing at the rDNA locus represses ncRNAs with a strength inversely proportional to Pol I transcription. Moreover, localized regions of histone hyperacetylation appear in cryptic promoter elements when Pol II is active and in the coding region when Pol I is functional; in addition, DNA topoisomerase I site-specific activity follows RNA polymerase I transcription. The repression of ncRNAs at the rDNA locus, in response to RNA polymerase I transcription, could represent a physiological circuit control whose mechanism involves modification of histone acetylation.

Growth control and ribosome biogenesis

Ribosomes provide the basis for protein production and this drives cell growth. Recent studies, both in yeast and in higher eukaryotes, are beginning to reveal new mechanisms underlying the elaborate control of ribosome biogenesis, which requires coordinate regulation of all three RNA polymerases. Transcription of ribosomal RNA is finely tuned to cellular energy status and linked to the production of ribosomal proteins. Several autoregulatory mechanisms controlling various aspects of ribosome biogenesis have been uncovered and reveal new connections to cell-cycle and cell-size control. Ribosome biogenesis has now been clearly linked to disease, particularly to cancer and anemia, and also to aging. A challenge for future studies will be to elucidate further the molecular mechanisms underlying these connections.

Loss of human ribosomal gene CpG methylation enhances cryptic RNA polymerase II transcription and disrupts ribosomal RNA processing

Epigenetic methyl-CpG silencing of the ribosomal RNA (rRNA) genes is thought to downregulate rRNA synthesis in mammals. In contrast, we now show that CpG methylation in fact positively influences rRNA synthesis and processing. Human HCT116 cells, inactivated for DNMT1 and DNMT3b or treated with aza-dC, lack CpG methylation and reactivate a large fraction of normally silent rRNA genes. Unexpectedly, these cells display reduced rRNA synthesis and processing and accumulate unprocessed 45S rRNA. Reactivation of the rRNA genes is associated with their cryptic transcription by RNA polymerase II. Ectopic expression of cryptic rRNA gene transcripts recapitulates the defects associated with loss of CpG methylation. The data demonstrate that rRNA gene silencing prevents cryptic RNA polymerase II transcription of these genes. Lack of silencing leads to the partial disruption of rRNA synthesis and rRNA processing, providing an explanation for the cytotoxic effects of loss of CpG methylation.

Opposing role of condensin and radiation-sensitive gene RAD52 in ribosomal DNA stability regulation

Blocking target of rapamycin signaling by starvation or rapamycin inhibits ribosomal DNA (rDNA) transcription and causes condensin-mediated rDNA condensation and nucleolar contraction. In the absence of condensin, however, repression of rDNA transcription leads to rDNA instability and elevated level of extrachromosomal rDNA circles and nucleolar fragmentation. Here, we show that mutations in the Rad52 homologous recombination machinery block rDNA instability. Rad52 is normally excluded from the nucleolus. In the absence of condensin, however, repression of rDNA transcription results in Rad52 localization to the nucleolus, association with rDNA and subsequent formation of extrachromosomal rDNA circles, and reduced cell survival. In contrast, deletion of RAD52 restores cell viability under the same conditions. These results reveal an opposing role of condensin and Rad52 in the control of rDNA stability under nutrient starvation conditions.

Genetic identification of factors that modulate ribosomal DNA transcription in Saccharomyces cerevisiae

Ribosomal RNA (rRNA) is transcribed from the ribosomal DNA (rDNA) genes by RNA polymerase I (Pol I). Despite being responsible for the majority of transcription in growing cells, Pol I regulation is poorly understood compared to Pol II. To gain new insights into rDNA transcriptional regulation, we developed a genetic assay in Saccharomyces cerevisiae that detects alterations in transcription from the centromere-proximal rDNA gene of the tandem array. Changes in Pol I transcription at this gene alter the expression of an adjacent, modified URA3 reporter cassette (mURA3) such that reductions in Pol I transcription induce growth on synthetic media lacking uracil. Increases in Pol I transcription induce growth on media containing 5-FOA. A transposon mutagenesis screen was performed with the reporter strain to identify genes that play a role in modulating rDNA transcription. Mutations in 68 different genes were identified, several of which were already known to function in chromatin modification and the regulation of Pol II transcription. Among the other classes of genes were those encoding proteasome subunits and multiple kinases and phosphatases that function in nutrient and stress signaling pathways. Fourteen genes were previously uncharacterized and have been named as regulators of rDNA transcription (RRT).

Limiting the extent of the RDN1 heterochromatin domain by a silencing barrier and Sir2 protein levels in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, transcriptional silencing occurs at the cryptic mating-type loci (HML and HMR), telomeres, and ribosomal DNA (rDNA; RDN1). Silencing in the rDNA is unusual in that polymerase II (Pol II) promoters within RDN1 are repressed by Sir2 but not Sir3 or Sir4. rDNA silencing unidirectionally spreads leftward, but the mechanism of limiting its spreading is unclear. We searched for silencing barriers flanking the left end of RDN1 by using an established assay for detecting barriers to HMR silencing. Unexpectedly, the unique sequence immediately adjacent to RDN1, which overlaps a prominent cohesin binding site (CARL2), did not have appreciable barrier activity. Instead, a fragment located 2.4 kb to the left, containing a tRNA(Gln) gene and the Ty1 long terminal repeat, had robust barrier activity. The barrier activity was dependent on Pol III transcription of tRNA(Gln), the cohesin protein Smc1, and the SAS1 and Gcn5 histone acetyltransferases. The location of the barrier correlates with the detectable limit of rDNA silencing when SIR2 is overexpressed, where it blocks the spreading of rDNA heterochromatin. We propose a model in which normal Sir2 activity results in termination of silencing near the physical rDNA boundary, while tRNA(Gln) blocks silencing from spreading too far when nucleolar Sir2 pools become elevated.

The Paf1 complex is required for efficient transcription elongation by RNA polymerase I

Regulation of RNA polymerase I (Pol I) transcription is critical for controlling ribosome synthesis. Most previous investigations into Pol I transcription regulation have focused on transcription initiation. To date, the factors involved in the control of Pol I transcription elongation are poorly understood. The Paf1 complex (Paf1C) is a well-defined factor that influences polymerase II (Pol II) transcription elongation. We found that Paf1C associates with rDNA. Deletion of genes for Paf1C subunits (CDC73, CTR9, or PAF1) reduces the rRNA synthesis rate; however, there is no significant alteration of rDNA copy number or Pol I occupancy of the rDNA. Furthermore, EM analysis revealed a substantial increase in the frequency of large gaps between transcribing polymerases in ctr9Delta mutant cells compared with WT. Together, these data indicate that Paf1C promotes Pol I transcription through the rDNA by increasing the net rate of elongation. Thus, the multifunctional, conserved transcription factor Paf1C plays an important role in transcription elongation by Pol I in vivo.

Pnc1p-mediated nicotinamide clearance modifies the epigenetic properties of rDNA silencing in Saccharomyces cerevisiae

The histone deacetylase activity of Sir2p is dependent on NAD(+) and inhibited by nicotinamide (NAM). As a result, Sir2p-regulated processes in Saccharomyces cerevisiae such as silencing and replicative aging are susceptible to alterations in cellular NAD(+) and NAM levels. We have determined that high concentrations of NAM in the growth medium elevate the intracellular NAD(+) concentration through a mechanism that is partially dependent on NPT1, an important gene in the Preiss-Handler NAD(+) salvage pathway. Overexpression of the nicotinamidase, Pnc1p, prevents inhibition of Sir2p by the excess NAM while maintaining the elevated NAD(+) concentration. This growth condition alters the epigenetics of rDNA silencing, such that repression of a URA3 reporter gene located at the rDNA induces growth on media that either lacks uracil or contains 5-fluoroorotic acid (5-FOA), an unusual dual phenotype that is reminiscent of telomeric silencing (TPE) of URA3. Despite the similarities to TPE, the modified rDNA silencing phenotype does not require the SIR complex. Instead, it retains key characteristics of typical rDNA silencing, including RENT and Pol I dependence, as well as a requirement for the Preiss-Handler NAD(+) salvage pathway. Exogenous nicotinamide can therefore have negative or positive impacts on rDNA silencing, depending on the PNC1 expression level.

Transcription of multiple yeast ribosomal DNA genes requires targeting of UAF to the promoter by Uaf30

Upstream activating factor (UAF) is a multisubunit complex that functions in the activation of ribosomal DNA (rDNA) transcription by RNA polymerase I (Pol I). Cells lacking the Uaf30 subunit of UAF reduce the rRNA synthesis rate by approximately 70% compared to wild-type cells and produce rRNA using both Pol I and Pol II. Miller chromatin spreads demonstrated that even though there is an overall reduction in rRNA synthesis in uaf30 mutants, the active rDNA genes in such strains are overloaded with polymerases. This phenotype was specific to defects in Uaf30, as mutations in other UAF subunits resulted in a complete absence of rDNA genes with high or even modest Pol densities. The lack of Uaf30 prevented UAF from efficiently binding to the rDNA promoter in vivo, leading to an inability to activate a large number of rDNA genes. The relatively few genes that did become activated were highly transcribed, apparently to compensate for the reduced rRNA synthesis capacity. The results show that Uaf30p is a key targeting factor for the UAF complex that facilitates activation of a large proportion of rDNA genes in the tandem array.

Actively transcribed rRNA genes in S. cerevisiae are organized in a specialized chromatin associated with the high-mobility group protein Hmo1 and are largely devoid of histone molecules

Synthesis of ribosomal RNAs (rRNAs) is the major transcriptional event in proliferating cells. In eukaryotes, ribosomal DNA (rDNA) is transcribed by RNA polymerase I from a multicopy locus coexisting in at least two different chromatin states. This heterogeneity of rDNA chromatin has been an obstacle to defining its molecular composition. We developed an approach to analyze differential protein association with each of the two rDNA chromatin states in vivo in the yeast Saccharomyces cerevisiae. We demonstrate that actively transcribed rRNA genes are largely devoid of histone molecules, but instead associate with the high-mobility group protein Hmo1.

Transcription termination and RNA degradation contribute to silencing of RNA polymerase II transcription within heterochromatin

Within the heterochromatin of budding yeast, RNA polymerase II (RNAPII) transcription is repressed by the Sir2 deacetylase. Although heterochromatic silencing is generally thought to be due to limited accessibility of the underlying DNA, there are several reports of RNAPII and basal transcription factors within silenced regions. Analysis of the rDNA array revealed cryptic RNAPII transcription within the "nontranscribed" spacer region. These transcripts are terminated by the Nrd1/Sen1 complex and degraded by the exosome. Mutations in this pathway lead to decreased silencing and dramatic chromatin changes in the rDNA locus. Interestingly, Nrd1 mutants also show higher levels of rDNA recombination, suggesting that the cryptic RNAPII transcription might have a physiological role in regulating rDNA copy number. The Nrd1/Sen1/exosome pathway also contributes to silencing at telomeric loci. These results suggest that silencing of heterochromatic genes in Saccharomyces cerevisiae occurs at both transcriptional and posttranscriptional levels.

RNA polymerase I transcription obstructs condensin association with 35S rRNA coding regions and can cause contraction of long repeat in Saccharomyces cerevisiae

In many eukaryotic cells, the ribosomal RNA gene (rDNA) is composed of a highly repetitive structure. Previously, we reported the isolation of condensin mutants of Saccharomyces cerevisiae that were defective in carrying long rDNA repeat due to the loss of the replication fork barrier (RFB) protein Fob1p; thus the repeat in the mutants were dramatically contracted. The reintroduction of the FOB1 gene suppressed the contraction of the repeat. It was found that condensin mainly localized at the RFB site in a FOB1-dependent fashion. Here, we show that RNA polymerase I transcription interferes with condensin association with 35S rRNA coding regions in fob1 cells and causes dramatic contraction of rDNA repeat in the fob1 condensin double mutant. Inactivation of RNA polymerase I suppresses the dramatic contraction of the rDNA repeat in the fob1 condensin double mutant. These results suggest that association of condensin with the RFB site outside the active transcription region avoids the dramatic contraction of the rDNA repeat. We also found that the stimulation of RNA polymerase II transcription within the rDNA repeat decreased condensin association with actively transcribed regions. Thus, a characteristic of condensin is that its association with the chromatin is interfered by transcription.

Nutrient starvation promotes condensin loading to maintain rDNA stability

Nutrient starvation or rapamycin treatment, through inhibition of target of rapamycin, causes condensation of ribosomal DNA (rDNA) array and nucleolar contraction in budding yeast. Here we report that under such conditions, condensin is rapidly relocated into the nucleolus and loaded to rDNA tandem repeats, which is required for rDNA condensation. Rpd3-dependent histone deacetylation is necessary and sufficient for condensin's relocalization and loading to rDNA array, suggesting that histone modification plays a regulatory role for condensin targeting. Rapamycin independently, yet coordinately, inhibits rDNA transcription and promotes condensin loading to rDNA array. Unexpectedly, we found that inhibition of rDNA transcription in the absence of condensin loading leads to rDNA instability. Our data suggest that enrichment of condensin prevents rDNA instability during nutrient starvation. Together, these observations unravel a novel role for condensin in the maintenance of regional genomic stability.