Rpb4, a non-essential subunit of core RNA polymerase II of Saccharomyces cerevisiae is important for activated transcription of a subset of genes

Structure of the recombinant RNA polymerase from African Swine Fever Virus

African Swine Fever Virus is a Nucleo-Cytoplasmic Large DNA Virus that causes an incurable haemorrhagic fever in pigs with a high impact on global food security. ASFV replicates in the cytoplasm of the infected cell and encodes its own transcription machinery that is independent of cellular factors, however, not much is known about how this system works at a molecular level. Here, we present methods to produce recombinant ASFV RNA polymerase, functional assays to screen for inhibitors, and high-resolution cryo-electron microscopy structures of the ASFV RNAP in different conformational states. The ASFV RNAP bears a striking resemblance to RNAPII with bona fide homologues of nine of its twelve subunits. Key differences include the fusion of the ASFV assembly platform subunits RPB3 and RPB11, and an unusual C-terminal domain of the stalk subunit vRPB7 that is related to the eukaryotic mRNA cap 2´-O-methyltransferase 1. Despite the high degree of structural conservation with cellular RNA polymerases, the ASFV RNAP is resistant to the inhibitors rifampicin and alpha-amanitin. The cryo-EM structures and fully recombinant RNAP system together provide an important tool for the design, development, and screening of antiviral drugs in a low biosafety containment environment.

Taf14 is required for the stabilization of transcription pre-initiation complex in Saccharomyces cerevisiae

Background

The YEATS domain is a highly conserved protein structure that interacts with acetylated and crotonylated lysine residues in N-terminal tails of histones. The budding yeast genome encodes three YEATS domain proteins (Taf14, Yaf9, and Sas5) that are all the subunits of different complexes involved in histone acetylation, gene transcription, and chromatin remodeling. As the strains deficient in all these three genes are inviable, it has been proposed that the YEATS domain is essential in yeast. In this study we investigate in more detail the requirement of YEATS domain proteins for yeast survival and the possible roles of Taf14 YEATS domain in the regulation of gene transcription.

Results

We found that YEATS domains are not essential for the survival of Saccharomyces cerevisiae cells. Although the full deletion of all YEATS proteins is lethal in yeast, we show that the viability of cells can be restored by the expression of the YEATS-less version of Taf14 protein. We also explore the in vivo functions of Taf14 protein and show that the primary role of its YEATS domain is to stabilize the transcription pre-initiation complex (PIC). Our results indicate that Taf14-mediated interactions become crucial for PIC formation in rpb9Δ cells, where the recruitment of TFIIF to the PIC is hampered. Although H3 K9 residue has been identified as the interaction site of the Taf14 YEATS domain in vitro, we found that it is not the only interaction target in vivo.

Conclusions

Lethality of YEATS-deficient cells can be rescued by the expression of truncated Taf14 protein lacking the entire YEATS domain, indicating that the YEATS domains are not required for cell survival. The YEATS domain of Taf14 participates in PIC stabilization and acetylated/crotonylated H3K9 is not the critical target of the Taf14 YEATS domain in vivo.

Rpb9-deficient cells are defective in DNA damage response and require histone H3 acetylation for survival

Rpb9 is a non-essential subunit of RNA polymerase II that is involved in DNA transcription and repair. In budding yeast, deletion of RPB9 causes several phenotypes such as slow growth and temperature sensitivity. We found that simultaneous mutation of multiple N-terminal lysines within histone H3 was lethal in rpb9Δ cells. Our results indicate that hypoacetylation of H3 leads to inefficient repair of DNA double-strand breaks, while activation of the DNA damage checkpoint regulators γH2A and Rad53 is suppressed in Rpb9-deficient cells. Combination of H3 hypoacetylation with the loss of Rpb9 leads to genomic instability, aberrant segregation of chromosomes in mitosis, and eventually to cell death. These results indicate that H3 acetylation becomes essential for efficient DNA repair and cell survival if a DNA damage checkpoint is defective.

The RNA Pol II sub-complex hsRpb4/7 is required for viability of multiple human cell lines

The evolutionarily conserved RNA Polymerase II Rpb4/7 sub-complex has been thoroughly studied in yeast and impacts gene expression at multiple levels including transcription, mRNA processing and decay. In addition Rpb4/7 exerts differential effects on gene expression in yeast and Rpb4 is not obligatory for yeast (S. cerevisiae) survival. Specialised roles for human (hs) Rpb4/7 have not been extensively described and we have probed this question by depleting hsRpb4/7 in established human cell lines using RNA interference. We find that Rpb4/7 protein levels are inter-dependent and accordingly, the functional effects of depleting either protein are co-incident. hsRpb4/7 exhibits gene-specific effects and cells initially remain viable upon hsRpb4/7 depletion. However prolonged hsRpb4/7 depletion is cytotoxic in the range of cell lines tested. Protracted cell death occurs by an unknown mechanism and in some cases is accompanied by a pronounced elongated cell morphology. In conclusion we provide evidence for a gene-specific role of hsRpb4/7 in human cell viability.

The eukaryotic transcriptional machinery regulates mRNA translation and decay in the cytoplasm

In eukaryotes, nuclear mRNA synthesis is physically separated from its cytoplasmic translation and degradation. Recent unexpected findings have revealed that, despite this separation, the transcriptional machinery can remotely control the cytoplasmic stages. Key to this coupling is the capacity of the transcriptional machinery to "imprint" the transcript with factors that escort it to the cytoplasm and regulate its localization, translation and decay. Some of these factors are known transcriptional regulators that also function in mRNA decay and are hence named "synthegradases". Imprinting can be carried out and/or regulated by RNA polymerase II or by promoter cis- and trans-acting elements. This article is part of a Special Issue entitled: RNA polymerase II Transcript Elongation.

The multifunctional Ccr4-Not complex directly promotes transcription elongation

The Ccr4-Not complex has been implicated in the control of multiple steps of mRNA metabolism; however, its functions in transcription remain ambiguous. The discovery that Ccr4/Pop2 is the major cytoplasmic mRNA deadenylase and the detection of Not proteins within mRNA processing bodies have raised questions about the roles of the Ccr4-Not complex in transcription. Here we firmly establish Ccr4-Not as a positive elongation factor for RNA polymerase II (RNAPII). The Ccr4-Not complex is targeted to the coding region of genes in a transcription-dependent manner similar to RNAPII and promotes elongation in vivo. Furthermore, Ccr4-Not interacts directly with elongating RNAPII complexes and stimulates transcription elongation of arrested polymerase in vitro. Ccr4-Not can reactivate backtracked RNAPII using a mechanism different from that of the well-characterized elongation factor TFIIS. While not essential for its interaction with elongation complexes, Ccr4-Not interacts with the emerging transcript and promotes elongation in a manner dependent on transcript length, although this interaction is not required for it to bind RNAPII. Our comprehensive analysis shows that Ccr4-Not directly regulates transcription, and suggests it does so by promoting the resumption of elongation of arrested RNAPII when it encounters transcriptional blocks in vivo.

Mediator subunits and histone methyltransferase Set2 contribute to Ino2-dependent transcriptional activation of phospholipid biosynthesis in the yeast Saccharomyces cerevisiae

To activate eukaryotic genes, several pathways which modify chromatin and recruit general factors of the transcriptional machinery are utilized. We investigated the factors required for activation of yeast phospholipid biosynthetic genes, depending on activator protein Ino2 which binds to the inositol/choline-responsive element (ICRE) upstream promoter motif together with its partner protein Ino4. We used a set of 15 strains each defective for one of the non essential subunits of yeast mediator complex and identified med2, med3, med15, med18 and med19 as impaired for inositol biosynthesis. In these mutants, ICRE-dependent gene activation was reduced to 13-22% of the wild-type level. We also demonstrate synthetic growth and activation defects among mediator mutants and mutants lacking defined histone modifications (snf1, gcn5) and transcriptional coactivators (sub1). Analysis of mutants defective for histone methylation (set1, set2 and dot1) and demethylation (jhd1, jhd2, gis1, rph1 and ecm5) revealed the importance of the H3 Lys36-specific Set2 methyltransferase for ICRE-dependent gene expression. Although defined mediator subunits are critical for gene activation, we could not detect their interaction with Ino2. In contrast, Ino2 directly binds to the Set2 histone methyltransferase. Mapping of interaction domains revealed the importance of the SET core domain which was necessary and sufficient for binding Ino2.

Identification of specific DNA binding residues in the TCP family of transcription factors in Arabidopsis

The TCP transcription factors control multiple developmental traits in diverse plant species. Members of this family share an approximately 60-residue-long TCP domain that binds to DNA. The TCP domain is predicted to form a basic helix-loop-helix (bHLH) structure but shares little sequence similarity with canonical bHLH domain. This classifies the TCP domain as a novel class of DNA binding domain specific to the plant kingdom. Little is known about how the TCP domain interacts with its target DNA. We report biochemical characterization and DNA binding properties of a TCP member in Arabidopsis thaliana, TCP4. We have shown that the 58-residue domain of TCP4 is essential and sufficient for binding to DNA and possesses DNA binding parameters comparable to canonical bHLH proteins. Using a yeast-based random mutagenesis screen and site-directed mutants, we identified the residues important for DNA binding and dimer formation. Mutants defective in binding and dimerization failed to rescue the phenotype of an Arabidopsis line lacking the endogenous TCP4 activity. By combining structure prediction, functional characterization of the mutants, and molecular modeling, we suggest a possible DNA binding mechanism for this class of transcription factors.

The dissociable RPB4 subunit of RNA Pol II has vital functions in Drosophila

RNA polymerase II (Pol II) is composed of a ten subunit core and a two subunit dissociable subcomplex comprising the fourth and seventh largest subunits, RPB4 and RPB7. The evolutionary highly conserved RPB4/7 heterodimer is positioned in the Pol II such that it can make contact with various factors involved in RNA biogenesis and is believed to play roles both during the process of transcription and post-transcription. A detailed analysis of RPB4/7 function in a multicellular eukaryote, however, is lacking partly because of the lack of a suitable genetic system. Here, we describe generation and initial analysis of Drosophila Rpb4 mutants. In the fly, RPB4 is a product of a bicistronic gene together with the ATAC histone acetyltransferase complex constituent ADA2a. DmAda2a and DmRpb4 are expressed during fly development at different levels. The structure of mature mRNA forms suggests that the production of DmADA2a and DmRPB4-specific mRNAs is ensured by alternative splicing. Genetic analysis indicates that both DmRPB4 and DmADA2a play essential roles, because their absence results in lethality in early and late larval stages, respectively. Upon stress of high temperature or nutritional starvation, the levels of RPB4 and ADA2a messages change differently. RPB4 colocalizes with Pol II to several sites on polytene chromosomes, however, at selected locus, the abundances of Pol II and RPB4 vary greatly. Our data suggest no tight functional link between DmADA2a and DmRPB4, and reveal differences in the abundances of Pol II core subunits and RPB4 localized at specific regions on polytene chromosomes, supporting the suggested role of RPB4 outside of transcription-engaged Pol II complexes.

Identification and characterization of DdRPB4, a subunit of Dictyostelium discoideum RNA polymerase II

Rpb4, the fourth largest subunit of the eukaryotic RNA polymerase II (RNAPII), is required for growth at extreme temperatures and for an appropriate response to nutrient starvation in yeast. Sequence homologs of Rpb4 are found in most sequenced genomes from yeast to humans. To elucidate the role of this subunit in nutrient starvation, we chose Dictyostelium discoideum, a soil amoeba, which responds to nutrient deprivation by undergoing a complex developmental program. Here we report the identification of homolog of Saccharomyces cerevisiae RPB4 in D. discoideum. Localization and complementation studies suggest that Rpb4 is functionally conserved. DdRPB4 transcript and protein levels are developmentally regulated. Although DdRPB4 could not be deleted, overexpression revealed that the Rpb4 protein is essential for cell survival and is regulated stringently at the post-transcriptional level in D. discoideum. Thus maintaining a critical level of Rpb4 is important for this organism.

Genomewide recruitment analysis of Rpb4, a subunit of polymerase II in Saccharomyces cerevisiae, reveals its involvement in transcription elongation

The Rpb4/Rpb7 subcomplex of yeast RNA polymerase II (Pol II) has counterparts in all multisubunit RNA polymerases from archaebacteria to higher eukaryotes. The Rpb4/7 subcomplex in Saccharomyces cerevisiae is unique in that it easily dissociates from the core, unlike the case in other organisms. The relative levels of Rpb4 and Rpb7 in yeasts affect the differential gene expression and stress response. Rpb4 is nonessential in S. cerevisiae and affects expression of a small number of genes under normal growth conditions. Here, using a chromatin immunoprecipitation ("ChIP on-chip") technique, we compared genomewide binding of Rpb4 to that of a core Pol II subunit, Rpb3. Our results showed that in spite of being nonessential for survival, Rpb4 was recruited on coding regions of most transcriptionally active genes, similar to the case with the core Pol II subunit, Rpb3, albeit to a lesser extent. The extent of Rpb4 recruitment increased with increasing gene length. We also observed Pol II lacking Rpb4 to be defective in transcribing long, GC-rich transcription units, suggesting a role for Rpb4 in transcription elongation. This role in transcription elongation was supported by the observed 6-azauracil (6AU) sensitivity of the rpb4Delta mutant. Unlike most phenotypes of rpb4Delta, the 6AU sensitivity of the rpb4Delta strain was not rescued by overexpression of RPB7. This report provides the first instance of a distinct role for Rpb4 in transcription, which is independent of its interacting partner, Rpb7.

Unstructured N terminus of the RNA polymerase II subunit Rpb4 contributes to the interaction of Rpb4.Rpb7 subcomplex with the core RNA polymerase II of Saccharomyces cerevisiae

Two subunits of eukaryotic RNA polymerase II, Rpb7 and Rpb4, form a subcomplex that has counterparts in RNA polymerases I and III. Although a medium resolution structure has been solved for the 12-subunit RNA polymerase II, the relative contributions of the contact regions between the subcomplex and the core polymerase and the consequences of disrupting them have not been studied in detail. We have identified mutations in the N-terminal ribonucleoprotein-like domain of Saccharomyces cerevisiae Rpb7 that affect its role in certain stress responses, such as growth at high temperature and sporulation. These mutations increase the dependence of Rpb7 on Rpb4 for interaction with the rest of the polymerase. Complementation analysis and RNA polymerase pulldown assays reveal that the Rpb4.Rbp7 subcomplex associates with the rest of the core RNA polymerase II through two crucial interaction points: one at the N-terminal ribonucleoprotein-like domain of Rpb7 and the other at the partially ordered N-terminal region of Rpb4. These findings are in agreement with the crystal structure of the 12-subunit polymerase. We show here that the weak interaction predicted for the N-terminal region of Rpb4 with Rpb2 in the crystal structure actually plays a significant role in interaction of the subcomplex with the core in vivo. Our mutant analysis also suggests that Rpb7 plays an essential role in the cell through its ability to interact with the rest of the polymerase.

Basal transcription machinery: role in regulation of stress response in eukaryotes

The holoenzyme of prokaryotic RNA polymerase consists of the core enzyme, made of two alpha, beta, beta' and omega subunits, which lacks promoter selectivity and a sigma (sigma) subunit which enables the core enzyme to initiate transcription in a promoter dependent fashion. A stress sigma factor sigma(s), in prokaryotes seems to regulate several stress response genes in conjunction with other stress specific regulators. Since the basic principles of transcription are conserved from simple bacteria to multicellular complex organisms, an obvious question is: what is the identity of a counterpart of sigma(s), that is closest to the core polymerase and that dictates transcription of stress regulated genes in general? In this review, we discuss the logic behind the suggestion that like in prokaryotes,eukaryotes also have a common functional unit in the transcription machinery through which the stress specific transcription factors regulate rapid and highly controlled induction of gene expression associated with generalized stress response and point to some candidates that would fit the bill of the eukaryotic sigma(s).

The fission yeast Rpb4 subunit of RNA polymerase II plays a specialized role in cell separation

RNA polymerase II is a complex of 12 subunits, Rpb1 to Rpb12, whose specific roles are only partly understood. Rpb4 is essential in mammals and fission yeast, but not in budding yeast. To learn more about the roles of Rpb4, we expressed the rpb4 gene under the control of regulatable promoters of different strength in fission yeast. We demonstrate that below a critical level of transcription, Rpb4 affects cellular growth proportional to its expression levels: cells expressing lower levels of rpb4 grew slower compared to cells expressing higher levels. Lowered rpb4 expression did not affect cell survival under several stress conditions, but it caused specific defects in cell separation similar to sep mutants. Microarray analysis revealed that lowered rpb4 expression causes a global reduction in gene expression, but the transcript levels of a distinct subset of genes were particularly responsive to changes in rpb4 expression. These genes show some overlap with those regulated by the Sep1-Ace2 transcriptional cascade required for cell separation. Most notably, the gene expression signature of cells with lowered rpb4 expression was highly similar to those of mcs6, pmh1, sep10 and sep15 mutants. Mcs6 and Pmh1 encode orthologs of metazoan TFIIH-associated cyclin-dependent kinase (CDK)-activating kinase (Cdk7-cyclin H-Mat1), while Sep10 and Sep15 encode mediator components. Our results suggest that Rpb4, along with some other general transcription factors, plays a specialized role in a transcriptional pathway that controls the cell cycle-regulated transcription of a specific subset of genes involved in cell division.

Crystal structure and RNA binding of the Rpb4/Rpb7 subunits of human RNA polymerase II

The Rpb4 and Rpb7 subunits of eukaryotic RNA polymerase II (RNAP(II)) form a heterodimer that protrudes from the 10-subunit core of the enzyme. We have obtained crystals of the human Rpb4/Rpb7 heterodimer and determined the structure to 2.7 A resolution. The presence of putative RNA-binding domains on the Rpb7 subunit and the position of the heterodimer close to the RNA exit groove in the 12 subunit yeast polymerase complex strongly suggests a role for the heterodimer in binding and stabilizing the nascent RNA transcript. We have complemented the structural analysis with biochemical studies directed at dissecting the RNA-binding properties of the human Rpb4/Rpb7 complex and that of the homologous E/F complex from Methanocaldococcus jannaschii. A number of conserved, solvent-exposed residues in both the human Rpb7 subunit and the archaeal E subunit have been modified by site-directed mutagenesis and the mutants tested for RNA binding by performing electrophoretic mobility shift assays. These studies have identified an elongated surface region on the corresponding face of both subunit E and Rpb7 that is involved in RNA binding. The area spans the nucleic acid binding face of the OB fold, including the B4-B5 loop, but also extends towards the N-terminal domain.

Rpb4 and Rpb7: A Sub-complex Integral to Multi-subunit RNA Polymerases Performs a Multitude of Functions

Rpb4 and Rpb7, are conserved subunits of RNA polymerase II that play important roles in stress responses such as growth at extreme temperatures, recovery from stationary phase, sporulation and pseudohyphal growth. Recent reports have shown that apart from stress response, these proteins also affect a multitude of processes including activated transcription, mRNA export, transcription coupled repair etc. We propose a model that integrates the multifarious roles of this sub-complex. We suggest that these proteins function by modulating interactions of one or more ancillary factors with the polymerase leading to specific transcription of subsets of these genes. Preliminary experimental evidence in support of such a model is discussed.

Glyceraldehyde-3-phosphate dehydrogenase and actin associate with RNA polymerase II and interact with its Rpb7 subunit

RNA polymerase II (pol II) purified from the fission yeast Schizosaccharomyces pombe was previously reported to be associated with the general transcription factor TFIIF and the C-terminal domain phosphatase Fcp1, as well as glyceraldehyde-3-phosphate dehydrogenase (GAPDH), which has recently been implicated in transcriptional activation in human cells. Here, we provide evidence that the Rpb7 subunit of pol II interacts with GAPDH. Two-hybrid screen identified GAPDH as an Rpb7-binding protein. In addition, GAPDH was affinity-purified from S. pombe extract by using an Rpb4/Rpb7-coupled column. We also identified actin as a pol II-associated protein and revealed the interaction between actin and Rpb7.

High-resolution protein-DNA contacts for the yeast RNA polymerase II general transcription machinery

We used site-specific protein-DNA photo-cross-linking to define contact points between Saccharomyces cerevisiae RNA polymerase II (RNAP II) and the general transcription factors TBP, TFIIB, and TFIIF on promoter DNA. We present three key findings: (i) the overall pattern of cross-link sites is remarkably similar between the yeast and the previously described human system, even though transcription initiates downstream of the DNA-TBP-TFIIB-RNAP II-TFIIF complex in the S. cerevisiae system; (ii) the yeast Rpb7 subunit of RNAP II forms strong and reproducible cross-links to both strands of promoter DNA; and (iii) a TFIIB arginine-78 to cysteine replacement (R78C), which shifts start site selection downstream of normal, does not affect TFIIB-DNA cross-links prior to promoter melting but instead affects downstream TFIIF-DNA interactions. These results support the premise that the overall structure of the RNAP II preinitiation complex is similar in all eukaryotes and imply that yeast RNAP II is able to scan template DNA downstream of the preinitiation complex for acceptable start sites. The novel Rpb7-DNA contact sites imply that either promoter DNA does not follow a straight path from TATA to the initiation site or the topology of Rpb7 within the DNA-TBP-TFIIB-RNAP II-TFIIF complex is different from that defined in the 12-subunit RNAP II X-ray structure. We discuss the implications of these results for the mechanism of preinitiation complex assembly and promoter melting.

Subcellular localization of RPB5-mediating protein and its putative functional partner

We previously identified a novel cellular protein, RPB5-mediating protein (RMP), that retains corepressor activity and functionally antagonizes transcriptional modulation via hepatitis B virus X protein. The subcellular localization of RMP was examined using green fluorescent protein-fused protein forms. We found that a nuclear localization signal (NLS) and a coiled-coil (CC) domain functioning as a cytoplasmic localization signal (CLS) are important for the subcellular localization of RMP. The CLS apparently acts dominantly, since RMP was mostly localized in the cytoplasm with weak and diffuse signals in the nucleus, and the NLS was indispensable for the nuclear localization of RMP only in the absence of the CLS. Using a yeast two-hybrid method, we isolated a putative corepressor, DNA methyltransferase 1-associating protein (DMAP1), which was found to bind to the CC domain of RMP. DMAP1 facilitated the nuclear localization of RMP and the corepressor activity of RMP in a dose-dependent manner by interacting with the CC domain of RMP. These results are discussed in light of a recent paper showing a novel evolutionarily conserved role of URI in the TOR signaling pathway.

Concerted assembly and cloning of multiple DNA segments using in vitro site-specific recombination: functional analysis of multi-segment expression clones

The ability to clone and manipulate DNA segments is central to molecular methods that enable expression, screening, and functional characterization of genes, proteins, and regulatory elements. We previously described the development of a novel technology that utilizes in vitro site-specific recombination to provide a robust and flexible platform for high-throughput cloning and transfer of DNA segments. By using an expanded repertoire of recombination sites with unique specificities, we have extended the technology to enable the high-efficiency in vitro assembly and concerted cloning of multiple DNA segments into a vector backbone in a predefined order, orientation, and reading frame. The efficiency and flexibility of this approach enables collections of functional elements to be generated and mixed in a combinatorial fashion for the parallel assembly of numerous multi-segment constructs. The assembled constructs can be further manipulated by directing exchange of defined segments with alternate DNA segments. In this report, we demonstrate feasibility of the technology and application to the generation of fusion proteins, the linkage of promoters to genes, and the assembly of multiple protein domains. The technology has broad implications for cell and protein engineering, the expression of multidomain proteins, and gene function analysis.

Domainal organization of the lower eukaryotic homologs of the yeast RNA polymerase II core subunit Rpb7 reflects functional conservation

The subcomplex of Rpb4 and Rpb7 subunits of RNA pol II in Saccharomyces cerevisiae is known to be an important determinant of transcription under a variety of physiological stresses. In S.cerevisiae, RPB7 is essential for cell viability while rpb4 null strains are temperature sensitive at low and high temperatures. The rpb4 null strain also shows defect in sporulation and a predisposed state of pseudohyphal growth. We show here that, apart from S.cerevisiae Rpb7, the Rpb7 homologs from other lower eukaryotes like Schizosaccharomyces pombe, Candida albicans and Dictyostelium discoideum can complement for the absence of S.cerevisiae RPB7. This is the first report where we have shown that both the C.albicans and D.discoideum homologs are functional orthologs of the yeast RPB7. We also show that high expression levels of S.cerevisiae RPB7 and its homologs rescue the sporulation defect of rpb4 homozygous null diploids, but only some of them cause significant enhancement of the pseudohyphal phenotype. Structural modeling of Rpb7 and its homologs show a high degree of conservation in the overall structure. This study indicates a structural and functional conservation of different Rpb7 across species and also a conserved role of Rpb7 in the subcomplex with respect to nutritional stress.

The Conserved and Non-conserved Regions of Rpb4 Are Involved in Multiple Phenotypes in Saccharomyces cerevisiae

Rpb4, the fourth largest subunit of RNA polymerase II in Saccharomyces cerevisiae, is required for many phenotypes, including growth at high and low temperatures, sporulation, pseudohyphal growth, activated transcription of a subset of genes, and efficient carbon and energy metabolism. We have used deletion analysis to delineate the domains of the protein involved in these multiple phenotypes. The scRpb4 protein is conserved at the N and C termini but possesses certain non-conserved regions in the central portion. Our deletion analysis and molecular modeling results show that the N- and C-terminal conserved regions of Rpb4 are involved in interaction with Rpb7, the Rpb4 interacting partner in the RNA polymerase II. We further show that the conserved N terminus is required for efficient activated transcription from the INO1 promoter but not the GAL10- or the HSE-containing promoters. The N terminus is not required for any of the stress responses tested: growth at high temperatures, sporulation, and pseudohyphal growth. The conserved C-terminal 23 amino acids are not required for the role of Rpb4 in the pseudohyphal growth phenotype but might play a role in other stress responses and activated transcription. From the deletion analysis of the non-conserved regions, we report that they influence phenotypes involving both the N and C termini (interaction with Rpb7 and transcription from the INO1 promoter) but not any of the stress-responsive phenotypes tested suggesting that they might be involved in maintaining the two conserved domains in an appropriate conformation for interaction with Rpb7 and other proteins. Taken together, our results allow us to assign phenotype-specific roles for the different conserved and non-conserved regions of Rpb4.

Structural and functional homology between the RNAP(I) subunits A14/A43 and the archaeal RNAP subunits E/F

In the archaeal RNA polymerase and the eukaryotic RNA polymerase II, two subunits (E/F and RPB4/RPB7, respectively) form a heterodimer that reversibly associates with the core of the enzyme. Recently it has emerged that this heterodimer also has a counterpart in the other eukaryotic RNA polymerases: in particular two subunits of RNA polymerase I (A14 and A43) display genetic and biochemical characteristics that are similar to those of the RPB4 and RPB7 subunits, despite the fact that only A43 shows some sequence homology to RPB7. We demonstrate that the sequence of A14 strongly suggests the presence of a HRDC domain, a motif that is found at the C-terminus of a number of helicases and RNases. The same motif is also seen in the structure of the F subunit, suggesting a structural link between A14 and the RPB4/C17/subunit F family, even in the absence of direct sequence homology. We show that it is possible to co-express and co-purify large amounts of the recombinant A14/A43 heterodimer, indicating a tight and specific interaction between the two subunits. To shed light on the function of the heterodimer, we performed gel mobility shift assays and showed that the A14/A43 heterodimer binds single-stranded RNA in a similar way to the archaeal E/F complex.

Architecture of initiation-competent 12-subunit RNA polymerase II

RNA polymerase (Pol) II consists of a 10-polypeptide catalytic core and the two-subunit Rpb4/7 complex that is required for transcription initiation. Previous structures of the Pol II core revealed a "clamp," which binds the DNA template strand via three "switch regions," and a flexible "linker" to the C-terminal repeat domain (CTD). Here we derived a model of the complete Pol II by fitting structures of the core and Rpb4/7 to a 4.2-A crystallographic electron density map. Rpb4/7 protrudes from the polymerase "upstream face," on which initiation factors assemble for promoter DNA loading. Rpb7 forms a wedge between the clamp and the linker, restricting the clamp to a closed position. The wedge allosterically prevents entry of the promoter DNA duplex into the active center cleft and induces in two switch regions a conformation poised for template-strand binding. Interaction of Rpb4/7 with the linker explains Rpb4-mediated recruitment of the CTD phosphatase to the CTD during Pol II recycling. The core-Rpb7 interaction and some functions of Rpb4/7 are apparently conserved in all eukaryotic and archaeal RNA polymerases but not in the bacterial enzyme.

Complete, 12-subunit RNA polymerase II at 4.1-A resolution: implications for the initiation of transcription

The x-ray structure of complete RNA polymerase II from Saccharomyces cerevisiae has been determined, including a heterodimer of subunits Rpb4 and Rpb7 not present in previous "core" polymerase II structures. The heterodimer maintains the polymerase in the conformation of a transcribing complex, may bind RNA as it emerges from the enzyme, and is in a position to interact with general transcription factors and the Mediator of transcriptional regulation.

Whole genome expression profiles of yeast RNA polymerase II core subunit, Rpb4, in stress and nonstress conditions

Organisms respond to environmental stress by adopting changes in gene expression at the transcriptional level. Rpb4, a nonessential subunit of the core RNA polymerase II has been proposed to play a role in non-stress-specific transcription and in the regulation of stress response in yeast. We find that in addition to the temperature sensitivity of the null mutant of Rpb4, diploid null mutants are also compromised in sporulation and show morphological changes associated with nitrogen starvation. Using whole genome expression analysis, we report here the effects of Rpb4 on expression of genes during normal growth and following heat shock and nutritional starvation. Our analysis shows that Rpb4 affects expression of a small yet significant fraction of the genome in both stress and normal conditions. We found that genes involved in galactose metabolism were dependent on the presence of Rpb4 irrespective of the environmental condition. Rpb4 was also found to affect the expression of several other genes specifically in conditions of nutritional starvation. The general defect in the absence of Rpb4 is in the expression of metabolic genes, especially those involved in carbon metabolism and energy generation. We report that various stresses are affected by RPB4 and that on overexpression the stress-specific activators can partially rescue the corresponding defects.

Structure of yeast RNA polymerase II in solution: implications for enzyme regulation and interaction with promoter DNA

An 18 A resolution structure of the 12-subunit yeast RNA polymerase II (RNAPII) calculated from electron microscope images of single particles preserved in amorphous ice reveals the conformation of the enzyme in solution. The Rpb4/Rpb7 polymerase subunit complex was localized and found to be ideally positioned to determine the path of the nascent RNA transcript. The RNAPII structure suggests a revised mode of interaction with promoter DNA and demonstrates that regulation of RNAPII must involve structural changes that render the enzyme competent for initiation.

Deletion of the RNA polymerase subunit RPB4 acts as a global, not stress-specific, shut-off switch for RNA polymerase II transcription at high temperatures

We used whole genome expression analysis to investigate the changes in the mRNA profile in cells lacking the Saccharomyces cerevisiae RNA polymerase II subunit RPB4 (Delta RPB4). Our results indicated that an essentially complete shutdown of transcription occurs upon temperature shift of this conditionally lethal mutant; 98% of mRNA transcript levels decrease at least 2-fold, 96% at least 4-fold. This data was supported by in vivo experiments that revealed a rapid and greater than 5-fold decline in steady state poly(A) RNA levels after the temperature shift. Expression of several individual genes, measured by Northern analysis, was also consistent with the whole genome expression profile. Finally we demonstrated that the loss of RNA polymerase II activity causes secondary effects on RNA polymerase I, but not RNA polymerase III, transcription. The transcription phenotype of the Delta RPB4 mutant closely mirrors that of the temperature-sensitive rpb1-1 mutant frequently implemented as a tool to inactivate the RNA polymerase II in vivo. Therefore, the Delta RPB4 mutant can be used to easily design strains that enable the study of distinct post-transcriptional cellular processes in the absence of RNA polymerase II transcription.

Structure of an archaeal homolog of the eukaryotic RNA polymerase II RPB4/RPB7 complex

The eukaryotic subunits RPB4 and RPB7 form a heterodimer that reversibly associates with the RNA polymerase II core and constitute the only two components of the enzyme for which no structural information is available. We have determined the crystal structure of the complex between the Methanococcus jannaschii subunits E and F, the archaeal homologs of RPB7 and RPB4. Subunit E has an elongated two-domain structure and contains two potential RNA binding motifs, while the smaller F subunit wraps around one side of subunit E, at the interface between the two domains. We propose a model for the interaction between RPB4/RPB7 and the core RNA polymerase in which the RNA binding face of RPB7 is positioned to interact with the nascent RNA transcript.