RNA polymerase I remains intact without subunit exchange through multiple rounds of transcription in Saccharomyces cerevisiae

Expression of RNA polymerase I catalytic core is influenced by RPA12

RNA Polymerase I (Pol I) has recently been recognized as a cancer therapeutic target. The activity of this enzyme is essential for ribosome biogenesis and is universally activated in cancers. The enzymatic activity of this multi-subunit complex resides in its catalytic core composed of RPA194, RPA135, and RPA12, a subunit with functions in RNA cleavage, transcription initiation and elongation. Here we explore whether RPA12 influences the regulation of RPA194 in human cancer cells. We use a specific small-molecule Pol I inhibitor BMH-21 that inhibits transcription initiation, elongation and ultimately activates the degradation of Pol I catalytic subunit RPA194. We show that silencing RPA12 causes alterations in the expression and localization of Pol I subunits RPA194 and RPA135. Furthermore, we find that despite these alterations not only does the Pol I core complex between RPA194 and RPA135 remain intact upon RPA12 knockdown, but the transcription of Pol I and its engagement with chromatin remain unaffected. The BMH-21-mediated degradation of RPA194 was independent of RPA12 suggesting that RPA12 affects the basal expression, but not the drug-inducible turnover of RPA194. These studies add to knowledge defining regulatory factors for the expression of this Pol I catalytic subunit.

Identification of an E3 ligase that targets the catalytic subunit of RNA Polymerase I upon transcription stress

RNA Polymerase I (Pol I) synthesizes rRNA, which is the first and rate-limiting step in ribosome biogenesis. Factors governing the stability of the polymerase complex are not known. Previous studies characterizing Pol I inhibitor BMH-21 revealed a transcriptional stress-dependent pathway for degradation of the largest subunit of Pol I, RPA194. To identify the E3 ligase(s) involved, we conducted a cell-based RNAi screen for ubiquitin pathway genes. We establish Skp-Cullin-F-box protein complex F-box protein FBXL14 as an E3 ligase for RPA194. We show that FBXL14 binds to RPA194 and mediates RPA194 ubiquitination and degradation in cancer cells treated with BMH-21. Mutation analysis in yeast identified lysines 1150, 1153, and 1156 on Rpa190 relevant for the protein degradation. These results reveal the regulated turnover of Pol I, showing that the stability of the catalytic subunit is controlled by the F-box protein FBXL14 in response to transcription stress.

Regulation of RNA Polymerase I Stability and Function

RNA polymerase I is a highly processive enzyme with fast initiation and elongation rates. The structure of Pol I, with its in-built RNA cleavage ability and incorporation of subunits homologous to transcription factors, enables it to quickly and efficiently synthesize the enormous amount of rRNA required for ribosome biogenesis. Each step of Pol I transcription is carefully controlled. However, cancers have highjacked these control points to switch the enzyme, and its transcription, on permanently. While this provides an exceptional benefit to cancer cells, it also creates a potential cancer therapeutic vulnerability. We review the current research on the regulation of Pol I transcription, and we discuss chemical biology efforts to develop new targeted agents against this process. Lastly, we highlight challenges that have arisen from the introduction of agents with promiscuous mechanisms of action and provide examples of agents with specificity and selectivity against Pol I.

Biogenesis of RNA Polymerases in Yeast

Eukaryotic RNA polymerases (RNA pols) transcriptional processes have been extensively investigated, and the structural analysis of eukaryotic RNA pols has been explored. However, the global assembly and biogenesis of these heteromultimeric complexes have been narrowly studied. Despite nuclear transcription being carried out by three RNA polymerases in eukaryotes (five in plants) with specificity in the synthesis of different RNA types, the biogenesis process has been proposed to be similar, at least for RNA pol II, to that of bacteria, which contains only one RNA pol. The formation of three different interacting subassembly complexes to conform the complete enzyme in the cytoplasm, prior to its nuclear import, has been assumed. In Saccharomyces cerevisiae, recent studies have examined in depth the biogenesis of RNA polymerases by characterizing some elements involved in the assembly of these multisubunit complexes, some of which are conserved in humans. This study reviews the latest studies governing the mechanisms and proteins described as being involved in the biogenesis of RNA polymerases in yeast.

Conditional depletion of the RNA polymerase I subunit PAF53 reveals that it is essential for mitosis and enables identification of functional domains

Our knowledge of the mechanism of rDNA transcription has benefited from the combined application of genetic and biochemical techniques in yeast. Nomura's laboratory (Nogi, Y., Vu, L., and Nomura, M. (1991) Proc. Natl. Acad. Sci. U.S.A. 88, 7026-7030 and Nogi, Y., Yano, R., and Nomura, M. (1991) Proc. Natl. Acad. Sci. U.S.A. 88, 3962-3966) developed a system in yeast to identify genes essential for ribosome biogenesis. Such systems have allowed investigators to determine whether a gene was essential and to determine its function in rDNA transcription. However, there are significant differences in both the structures and components of the transcription apparatus and the patterns of regulation between mammals and yeast. Thus, there are significant deficits in our understanding of mammalian rDNA transcription. We have developed a system combining CRISPR/Cas9 and an auxin-inducible degron that enables combining a "genetics-like"approach with biochemistry to study mammalian rDNA transcription. We now show that the mammalian orthologue of yeast RPA49, PAF53, is required for rDNA transcription and mitotic growth. We have studied the domains of the protein required for activity. We have found that the C-terminal, DNA-binding domain (tandem-winged helix), the heterodimerization, and the linker domain were essential. Analysis of the linker identified a putative helix-turn-helix (HTH) DNA-binding domain. This HTH constitutes a second DNA-binding domain within PAF53. The HTH of the yeast and mammalian orthologues is essential for function. In summary, we show that an auxin-dependent degron system can be used to rapidly deplete nucleolar proteins in mammalian cells, that PAF53 is necessary for rDNA transcription and cell growth, and that all three PAF53 domains are necessary for its function.

Small-Molecule Targeting of RNA Polymerase I Activates a Conserved Transcription Elongation Checkpoint

Inhibition of RNA polymerase I (Pol I) is a promising strategy for modern cancer therapy. BMH-21 is a first-in-class small molecule that inhibits Pol I transcription and induces degradation of the enzyme, but how this exceptional response is enforced is not known. Here, we define key elements requisite for the response. We show that Pol I preinitiation factors and polymerase subunits (e.g., RPA135) are required for BMH-21-mediated degradation of RPA194. We further find that Pol I inhibition and induced degradation by BMH-21 are conserved in yeast. Genetic analyses demonstrate that mutations that induce transcription elongation defects in Pol I result in hypersensitivity to BMH-21. Using a fully reconstituted Pol I transcription assay, we show that BMH-21 directly impairs transcription elongation by Pol I, resulting in long-lived polymerase pausing. These studies define a conserved regulatory checkpoint that monitors Pol I transcription and is activated by therapeutic intervention.

The nuclear physique

This volume brings together a number of perspectives on how certain physical phenomena contribute to the functional design and operation of the nucleus. This collection could not be more timely, resonating with an increasing awareness of the opportunities that lie at the interface of cell biology and the physical sciences. For example, this was a major theme in the 2012 and 2013 annual meetings of the American Society for Cell Biology, and one that the Society aims to emphasize even further going forward. In addition, the emerging canonical relevance of the physical sciences to cell biology has in recent summers made a most conspicuous appearance in the curriculum (lectures and intense labs) of the famed Physiology Course at the Marine Biological Laboratory in Woods Hole. So, much credit is due to Ronald Hancock and Kwang Jeon, the coeditors of this volume, and all the authors for creating a work that is so au courant.

RNA polymerase II subunits exhibit a broad distribution of macromolecular assembly states in the interchromatin space of cell nuclei

Nearly all cellular processes are enacted by multi-subunit protein complexes, yet the assembly mechanism of most complexes is not well understood. The anthropomorphism "protein recruitment" that is used to describe the concerted binding of proteins to accomplish a specific function conceals significant uncertainty about the underlying physical phenomena and chemical interactions governing the formation of macromolecular complexes. We address this deficiency by investigating the diffusion dynamics of two RNA polymerase II subunits, Rpb3 and Rpb9, in regions of live Drosophila cell nuclei that are devoid of chromatin binding sites. Using FRAP microscopy, we demonstrate that both unengaged subunits are incorporated into a broad distribution of complexes, with sizes ranging from free (unincorporated) proteins to those that have been predicted for fully assembled gene transcription units. In live cells, Rpb3 exhibits regions of stability at both size extremes connected by a continuous distribution of complexes. Corresponding measurements on cellular extracts reveal a distribution that retains peaks at the extremes but not in between, suggesting that partially assembled complexes are less stable. We propose that the broad distribution of macromolecular species allows for mechanistic flexibility in the assembly of transcription complexes.

Quantifying transcription factor kinetics: at work or at play?

Transcription factors (TFs) interact dynamically in vivo with chromatin binding sites. Here we summarize and compare the four different techniques that are currently used to measure these kinetics in live cells, namely fluorescence recovery after photobleaching (FRAP), fluorescence correlation spectroscopy (FCS), single molecule tracking (SMT) and competition ChIP (CC). We highlight the principles underlying each of these approaches as well as their advantages and disadvantages. A comparison of data from each of these techniques raises an important question: do measured transcription kinetics reflect biologically functional interactions at specific sites (i.e. working TFs) or do they reflect non-specific interactions (i.e. playing TFs)? To help resolve this dilemma we discuss five key unresolved biological questions related to the functionality of transient and prolonged binding events at both specific promoter response elements as well as non-specific sites. In support of functionality, we review data suggesting that TF residence times are tightly regulated, and that this regulation modulates transcriptional output at single genes. We argue that in addition to this site-specific regulatory role, TF residence times also determine the fraction of promoter targets occupied within a cell thereby impacting the functional status of cellular gene networks. Thus, TF residence times are key parameters that could influence transcription in multiple ways.

Odd RNA polymerases or the A(B)C of eukaryotic transcription

Pioneering studies on eukaryotic transcription were undertaken with the bacterial system in mind. Will the bacterial paradigm apply to eukaryotes? Are there promoter sites scattered in the eukaryotic genome, and sigma-like proteins? Why three forms of RNA polymerase in eukaryotic cells? Why are they structurally so complex, in particular RNA polymerases I and III, compared to the bacterial enzyme? These questions and others that were raised along the way are evoked in this short historical survey of odd RNA polymerases studies, with some emphasis on the contribution of these studies to our global understanding of eukaryotic transcription systems. This article is part of a Special Issue entitled: Transcription by Odd Pols.

The trypanosomatid-specific N terminus of RPA2 is required for RNA polymerase I assembly, localization, and function

African trypanosomes are the only organisms known to use RNA polymerase I (pol I) to transcribe protein-coding genes. These genes include VSG, which is essential for immune evasion and is transcribed from an extranucleolar expression site body (ESB). Several trypanosome pol I subunits vary compared to their homologues elsewhere, and the question arises as to how these variations relate to pol I function. A clear example is the N-terminal extension found on the second-largest subunit of pol I, RPA2. Here, we identify an essential role for this region. RPA2 truncation leads to nuclear exclusion and a growth defect which phenocopies single-allele knockout. The N terminus is not a general nuclear localization signal (NLS), however, and it fails to accumulate unrelated proteins in the nucleus. An ectopic NLS is sufficient to reinstate nuclear localization of truncated RPA2, but it does not restore function. Moreover, NLS-tagged, truncated RPA2 has a different subnuclear distribution to full-length protein and is unable to build stable pol I complexes. We conclude that the RPA2 N-terminal extension does not have a role exclusive to the expression of protein-coding genes, but it is essential for all pol I functions in trypanosomes because it directs trypanosomatid-specific interactions with RPA1.

HSP90 and the R2TP co-chaperone complex: building multi-protein machineries essential for cell growth and gene expression

HSP90 (Heat Shock Protein 90) is an essential chaperone involved in the last folding steps of client proteins. It has many clients, and these are often recognized through specific adaptors. Recently, the conserved R2TP complex was identified as a key HSP90 co-chaperone. Current evidences indicate that the HSP90/R2TP system assembles multi-molecular protein complexes. Strikingly, these comprise basic machineries of gene expression: (1) nuclear RNA polymerases; (2) the snoRNPs, essential to produce ribosomes; and (3) mTOR Complex 1 and 2, which control translational activity and cell growth. Another important substrate is the telomerase RNP, required for continuous cell proliferation. We discuss here the assembly of RNA polymerases in bacteria and eukaryotes, the role of HSP90/R2TP in this process and in the assembly of snoRNPs and the PIKK family of TORC1 kinase. Finally, we speculate on the roles of R2TP as a master regulator of cell growth under normal or pathological conditions.

Biogenesis of multisubunit RNA polymerases

Gene transcription in the nucleus of eukaryotic cells is carried out by three related multisubunit RNA polymerases, Pol I, Pol II and Pol III. Although the structure and function of the polymerases have been studied extensively, little is known about their biogenesis and their transport from the cytoplasm (where the subunits are synthesized) to the nucleus. Recent studies have revealed polymerase assembly intermediates and putative assembly factors, as well as factors required for Pol II nuclear import. In this review, we integrate the available data into a model of Pol II biogenesis that provides a framework for future analysis of the biogenesis of all RNA polymerases.

Assembly of the transcription machinery: ordered and stable, random and dynamic, or both?

The assembly of the transcription machinery is a key step in gene activation, but even basic details of this process remain unclear. Here we discuss the apparent discrepancy between the classic sequential assembly model based mostly on biochemistry and an emerging dynamic assembly model based mostly on fluorescence microscopy. The former model favors a stable transcription complex with subunits that cooperatively assemble in order, whereas the latter model favors an unstable complex with subunits that may assemble more randomly. To confront this apparent discrepancy, we review the merits and drawbacks of the different experimental approaches and list potential biasing factors that could be responsible for the different interpretations of assembly. We then discuss how these biases might be overcome in the future with improved experiments or new techniques. Finally, we discuss how kinetic models for assembly may help resolve the ordered and stable vs. random and dynamic assembly debate.

Imaging transcription in living cells

The advent of new technologies for the imaging of living cells has made it possible to determine the properties of transcription, the kinetics of polymerase movement, the association of transcription factors, and the progression of the polymerase on the gene. We report here the current state of the field and the progress necessary to achieve a more complete understanding of the various steps in transcription. Our Consortium is dedicated to developing and implementing the technology to further this understanding.

Modulation of RNA polymerase assembly dynamics in transcriptional regulation

The interaction of transcription factors with target genes is highly dynamic. Whether the dynamic nature of these interactions is merely an intrinsic property of transcription factors or serves a regulatory role is unknown. Here we have used single-cell fluorescence imaging combined with computational modeling and chromatin immunoprecipitation to analyze transcription complex dynamics in gene regulation during the cell cycle in living cells. We demonstrate a link between the dynamics of RNA polymerase I (RNA Pol I) assembly and transcriptional output. We show that transcriptional upregulation is accompanied by prolonged retention of RNA Pol I components at the promoter, resulting in longer promoter dwell time, and an increase in the steady-state population of assembling polymerase. As a consequence, polymerase assembly efficiency and, ultimately, the rate of entry into processive elongation are elevated. Our results show that regulation of rDNA transcription in vivo occurs via modulation of the efficiency of transcription complex subunit capture and assembly.

Beyond the sequence: cellular organization of genome function

Genomes are more than linear sequences. In vivo they exist as elaborate physical structures, and their functional properties are strongly determined by their cellular organization. I discuss here the functional relevance of spatial and temporal genome organization at three hierarchical levels: the organization of nuclear processes, the higher-order organization of the chromatin fiber, and the spatial arrangement of genomes within the cell nucleus. Recent insights into the cell biology of genomes have overturned long-held dogmas and have led to new models for many essential cellular processes, including gene expression and genome stability.

What are the molecular ties that maintain genomic loops?

The formation of genomic loops by proteins bound at sites scattered along a chromosome has a central role in many cellular processes, such as transcription, recombination and replication. Until recently, few such loops had been analyzed in any detail, and there was little agreement about the nature of the molecular ties maintaining these loops. Recent evidence suggests that loops are found in both prokaryotes and eukaryotes, and that the transcription machinery is a molecular tie. In addition, results obtained using site-specific recombination in bacteria and chromosome conformation capture in eukaryotes support the idea that active transcription units are in close contact. These data are consistent with a model for genome organization in which active polymerases cluster into transcription 'factories', which, inevitably, loops the intervening DNA. They are also consistent with the ties functioning as barriers, silencers, enhancers or locus control regions, depending on their positions relative to other genes.

RNA polymerase II elongation factors Spt4p and Spt5p play roles in transcription elongation by RNA polymerase I and rRNA processing

Previous investigations into the mechanisms that control RNA Polymerase (Pol) I transcription have primarily focused on the process of transcription initiation, thus little is known regarding postinitiation steps in the transcription cycle. Spt4p and Spt5p are conserved throughout eukaryotes, and they affect elongation by Pol II. We have found that these two proteins copurify with Pol I and associate with the rDNA in vivo. Disruption of the gene for Spt4p resulted in a modest decrease in growth and rRNA synthesis rates at the permissive temperature, 30 degrees C. Furthermore, biochemical and EM analyses showed clear defects in rRNA processing. These data suggest that Spt4p, Spt5p, and, potentially, other regulators of Pol I transcription elongation play important roles in coupling rRNA transcription to its processing and ribosome assembly.

Structure and function of the nucleolus in the spotlight

The nucleolus is the most obvious and clearly differentiated nuclear sub-compartment. It is where ribosome biogenesis takes place, but it is becoming clear that the nucleolus also has non-ribosomal functions. In this review we discuss recent progress in our understanding of how both ribosome biosynthesis and some non-ribosomal functions relate to observable nucleolar structure. We still do not have detailed enough information about the in situ organization of the various processes taking place in the nucleolus. However, the present power of light and electron microscopy techniques means that a description of the organization of nucleolar processes at the molecular level is now achievable, and the time is ripe for such an effort.

New Insights into Nucleolar Architecture and Activity

The nucleolus is the most obvious and clearly differentiated nuclear subcompartment. It is where ribosome biogenesis takes place and has been the subject of research over many decades. In recent years progress in our understanding of ribosome biogenesis has been rapid and is accelerating. This review discusses current understanding of how the biochemical processes of ribosome biosynthesis relate to an observable nucleolar structure. Emerging evidence is also described that points to other, unconventional roles for the nucleolus, particularly in the biogenesis of other RNA-containing cellular machinery, and in stress sensing and the control of cellular activity. Striking recent observations show that the nucleolus and its components are highly dynamic, and that the steady state structure observed by microscopical methods must be interpreted as the product of these dynamic processes. We still do not have detailed enough information to understand fully the organization and regulation of the various processes taking place in the nucleolus. However, the present power of light and electron microscopy (EM) techniques means that a description of nucleolar processes at the molecular level is now achievable, and the time is ripe for such an effort.

Nucleolar biogenesis: the first small steps

The nucleolus is the site of rRNA transcription, pre-rRNA processing and ribosome subunit assembly. The nucleolus assembles around clusters of ribosomal gene repeats during late telophase, persists throughout interphase and then disassembles as cells enter mitosis. The initial step in nucleolar formation is ribosomal gene transcription, which is mediated by Pol I (RNA polymerase I) and its associated transcription factors: UBF (upstream-binding factor), SL1 (selectivity factor) and TIF-IA (transcription initiation factor IA)/Rrn3. Ribosomal gene clusters, termed NORs (nucleolar organizer regions), are found on each of the five human acrocentric chromosomes. Though transcription is repressed during metaphase, NORs that were active in the previous interphase form prominent cytogenetic features, namely secondary constrictions. The main defining characteristic of these constrictions is under-condensation in comparison with the rest of the chromosome. Extensive binding of UBF over the ribosomal gene repeat is responsible for the formation of this chromosomal feature. During interphase, the majority of the Pol I transcription machinery, though present in nucleoli, is not actively engaged in transcription. Interaction with UBF bound across the gene repeat provides an explanation for how this non-engaged Pol I machinery is sequestered by nucleoli.

The moving parts of the nucleolus

The cell nucleolus is the subnuclear body in which ribosomal subunits are assembled, and it is also the location of several processes not related to ribosome biogenesis. Recent studies have revealed that nucleolar components move about in a variety of ways. One class of movement is associated with ribosome assembly, which is a vectorial process originating at the sites of transcription in the border region between the fibrillar center and the dense fibrillar component. The nascent preribosomal particles move outwardly to become the granular components where further maturation takes place. These particles continue their travel through the nucleoplasm for eventual export to the cytoplasm to become functional ribosomes. In a second kind of motion, many nucleolar components rapidly exchange with the nucleoplasm. Thirdly, nucleolar components engage in very complex movements when the nucleolus disassembles at the beginning of mitosis and then reassembles at the end of mitosis. Finally, many other cellular and viral macromolecules, which are not related to ribosome assembly, also pass through or are retained by the nucleolus. These are involved in nontraditional roles of the nucleolus, including regulation of tumor suppressor and oncogene activities, signal recognition particle assembly, modification of small RNAs, control of aging, and modulating telomerase function.