A suppressor of an RNA polymerase II mutation of Saccharomyces cerevisiae encodes a subunit common to RNA polymerases I, II, and III

De novo variants in POLR3B cause ataxia, spasticity, and demyelinating neuropathy

POLR3B encodes the second-largest catalytic subunit of RNA polymerase III, an enzyme involved in transcription. Bi-allelic pathogenic variants in POLR3B are a well-established cause of hypomyelinating leukodystrophy. We describe six unrelated individuals with de novo missense variants in POLR3B and a clinical presentation substantially different from POLR3-related leukodystrophy. These individuals had afferent ataxia, spasticity, variable intellectual disability and epilepsy, and predominantly demyelinating sensory motor peripheral neuropathy. Protein modeling and proteomic analysis revealed a distinct mechanism of pathogenicity; the de novo POLR3B variants caused aberrant association of individual enzyme subunits rather than affecting overall enzyme assembly or stability. We expand the spectrum of disorders associated with pathogenic variants in POLR3B to include a de novo heterozygous POLR3B-related disorder.

Sub1 contacts the RNA polymerase II stalk to modulate mRNA synthesis

Biogenesis of messenger RNA is critically influenced by the phosphorylation state of the carboxy-terminal domain (CTD) in the largest RNA polymerase II (RNAPII) subunit. Several kinases and phosphatases are required to maintain proper CTD phosphorylation levels and, additionally, several other proteins modulate them, including Rpb4/7 and Sub1. The Rpb4/7 heterodimer, constituting the RNAPII stalk, promote phosphatase functions and Sub1 globally influences CTD phosphorylation, though its mechanism remains mostly unknown. Here, we show that Sub1 physically interacts with the RNAPII stalk domain, Rpb4/7, likely through its C-terminal region, and associates with Fcp1. While Rpb4 is not required for Sub1 interaction with RNAPII complex, a fully functional heterodimer is required for Sub1 association to promoters. We also demonstrate that a complete CTD is necessary for proper association of Sub1 to chromatin and to the RNAPII. Finally, genetic data show a functional relationship between Sub1 and the RNAPII clamp domain. Altogether, our results indicate that Sub1, Rpb4/7 and Fcp1 interaction modulates CTD phosphorylation. In addition, Sub1 interaction with Rpb4/7 can also modulate transcription start site selection and transcription elongation rate likely by influencing the clamp function.

Two Routes to Genetic Suppression of RNA Trimethylguanosine Cap Deficiency via C-Terminal Truncation of U1 snRNP Subunit Snp1 or Overexpression of RNA Polymerase Subunit Rpo26

The trimethylguanosine (TMG) caps of small nuclear (sn) RNAs are synthesized by the enzyme Tgs1 via sequential methyl additions to the N2 atom of the m⁷G cap. Whereas TMG caps are inessential for Saccharomyces cerevisiae vegetative growth at 25° to 37°, tgs1∆ cells that lack TMG caps fail to thrive at 18°. The cold-sensitive defect correlates with ectopic stoichiometric association of nuclear cap-binding complex (CBC) with the residual m⁷G cap of the U1 snRNA and is suppressed fully by Cbc2 mutations that weaken cap binding. Here, we show that normal growth of tgs1∆ cells at 18° is also restored by a C-terminal deletion of 77 amino acids from the Snp1 subunit of yeast U1 snRNP. These results underscore the U1 snRNP as a focal point for TMG cap function in vivo. Casting a broader net, we conducted a dosage suppressor screen for genes that allowed survival of tgs1∆ cells at 18°. We thereby recovered RPO26 (encoding a shared subunit of all three nuclear RNA polymerases) and RPO31 (encoding the largest subunit of RNA polymerase III) as moderate and weak suppressors of tgs1∆ cold sensitivity, respectively. A structure-guided mutagenesis of Rpo26, using rpo26∆ complementation and tgs1∆ suppression as activity readouts, defined Rpo26-(78-155) as a minimized functional domain. Alanine scanning identified Glu89, Glu124, Arg135, and Arg136 as essential for rpo26∆ complementation. The E124A and R135A alleles retained tgs1∆ suppressor activity, thereby establishing a separation-of-function. These results illuminate the structure activity profile of an essential RNA polymerase component.

Site specific phosphorylation of yeast RNA polymerase I

All nuclear RNA polymerases are phosphoprotein complexes. Yeast RNA polymerase I (Pol I) contains approximately 15 phosphate groups, distributed to 5 of the 14 subunits. Information about the function of the single phosphosites and their position in the primary, secondary and tertiary structure is lacking. We used a rapid and efficient way to purify yeast RNA Pol I to determine 13 phosphoserines and –threonines. Seven of these phosphoresidues could be located in the 3D-homology model for Pol I, five of them are more at the surface. The single phosphorylated residues were systematically mutated and the resulting strains and Pol I preparations were analyzed in cellular growth, Pol I composition, stability and genetic interaction with non-essential components of the transcription machinery. Surprisingly, all Pol I phosphorylations analyzed were found to be non-essential post-translational modifications. However, one mutation (subunit A190 S685D) led to higher growth rates in the presence of 6AU or under environmental stress conditions, and was synthetically lethal with a deletion of the Pol I subunit A12.2, suggesting a role in RNA cleavage/elongation or termination. Our results suggest that individual major or constitutively phosphorylated residues contribute to non-essential Pol I-functions.

Reconstitution of transcription from the human U6 small nuclear RNA promoter with eight recombinant polypeptides and a partially purified RNA polymerase III complex

The human U6 small nuclear (sn) RNA core promoter consists of a proximal sequence element, which recruits the multisubunit factor SNAP(c), and a TATA box, which recruits the TATA box-binding protein, TBP. In addition to SNAP(c) and TBP, transcription from the human U6 promoter requires two well defined factors. The first is hB", a human homologue of the B" subunit of yeast TFIIIB generally required for transcription of RNA polymerase III genes, and the second is hBRFU, one of two human homologues of the yeast TFIIIB subunit BRF specifically required for transcription of U6-type RNA polymerase III promoters. Here, we have partially purified and characterized a RNA polymerase III complex that can direct transcription from the human U6 promoter when combined with recombinant SNAP(c), recombinant TBP, recombinant hB", and recombinant hBRFU. These results open the way to reconstitution of U6 transcription from entirely defined components.

Isolation and characterization of monoclonal antibodies directed against subunits of human RNA polymerases I, II, and III

Human nuclei contain three different RNA polymerases: polymerases I, II, and III. Each polymerase is a multi-subunit enzyme with 12-17 subunits. The localization of these subunits is limited by the paucity of antibodies suitable for immunofluorescence. We now describe eight different monoclonal antibodies that react specifically with RPB6 (also known as RPA20, RPB14.4, or RPC20), RPB8 (RPA18, RPB17, or RPC18), RPC32, or RPC39 and which are suitable for such studies. Each antibody detects one specific band in immunoblots of nuclear extracts; each also immunoprecipitates large complexes containing many other subunits. When used for immunofluorescence, antibodies against the subunits shared by all three polymerases (i.e., RPB6, RPB8) gave a few bright foci in nucleoli and nucleoplasm, as well as many fainter nucleoplasmic foci; all the bright foci were generally distinct from speckles containing Sm antigen. Antibodies against the two subunits found only in polymerase III (i.e., RPC32, RPC39) gave a few bright and many faint nucleoplasmic foci, but no nucleolar foci. Growth in two transcriptional inhibitors-5, 6-dichloro-1-beta-d-ribofuranosylbenzimidazole and actinomycin D-led to the redistribution of each subunit in a characteristic manner.

Mutants in ABC10beta, a conserved subunit shared by all three yeast RNA polymerases, specifically affect RNA polymerase I assembly

ABC10beta, a small polypeptide common to the three yeast RNA polymerases, has close homology to the N subunit of the archaeal enzyme and is remotely related to the smallest subunit of vaccinial RNA polymerase. The eucaryotic, archaeal, and viral polypeptides share an invariant motif CX2C. CC that is strictly essential for yeast growth, as shown by site-directed mutagenesis, whereas the rest of the ABC10beta sequence is fairly tolerant to amino acid replacements. ABC10beta has Zn2+ binding properties in vitro, and the CX2C. CC motif may therefore define an atypical metal-chelating site. Hybrid subunits that derive most of their amino acids from the archaeal subunit are functional in yeast, indicating that the archaeal and eucaryotic polypeptides have a largely equivalent role in the organization of their respective transcription complexes. However, all eucaryotic forms of ABC10beta harbor a HVDLIEK motif that, when mutated or replaced by its archaeal counterpart, leads to a polymerase I-specific lethal defect in vivo. This is accompanied by a specific lack in the largest subunit of RNA polymerase I (A190) in cell-free extracts, showing that the mutant enzyme is not properly assembled in vivo.

Genetic evidence for selective degradation of RNA polymerase subunits by the 20S proteasome in Saccharomyces cerevisiae

scs32 was isolated as an extragenic suppressor of a temperature-sensitive (ts) mutation (rpo26-31) in the gene encoding Rpo26p, a subunit common to yeast nuclear RNA polymerases (RNAPs). rpo26-31 also confers inositol auxotrophy, inhibits the assembly of RNAPI and RNAPII and reduces the steady-state level of Rpo26p and the largest subunit of RNAPI (Rpo11p or A190p) and RNAPII (Rpo21p). rpo26-31p accumulated to wild-type levels in the scs32 strain; nevertheless, the amount of assembled RNAPII remained at a reduced level at high temperature. Hence, scs32 only partially suppressed the ts phenotype and was unable to suppress the Ino-phenotype of rpo26-31. SCS32 is identical to PUP3, which encodes a subunit of the yeast proteasome. scs32 was able to suppress the phenotype of other ts alleles of RPO26, all of which reduce the steady-state level of this subunit. However, scs32 was unable to suppress the ts phenotype of mutant alleles of RPO21, or result in accumulation of the unstable rpo21-4p. These observations suggest that the stability of non-functional or unassembled forms of Rpo26p and Rpo21p are regulated independently.

Interactions between the human RNA polymerase II subunits

As an initial approach to characterizing the molecular structure of the human RNA polymerase II (hRPB), we systematically investigated the protein-protein contacts that the subunits of this enzyme may establish with each other. To this end, we applied a glutathione S-transferase-pulldown assay to extracts from Sf9 insect cells, which were coinfected with all possible combinations of recombinant baculoviruses expressing hRPB subunits, either as untagged polypeptides or as glutathione S-transferase fusion proteins. This is the first comprehensive study of interactions between eukaryotic RNA polymerase subunits; among the 116 combinations of hRPB subunits tested, 56 showed significant to strong interactions, whereas 60 were negative. Within the intricate network of interactions, subunits hRPB3 and hRPB5 play a central role in polymerase organization. These subunits, which are able to homodimerize and to interact, may constitute the nucleation center for polymerase assembly, by providing a large interface to most of the other subunits.

Mutations in an Abf1p binding site in the promoter of yeast RPO26 shift the transcription start sites and reduce the level of RPO26 mRNA

A binding site for the transcription factor Abf1p was identified as an important promoter element of the gene that encodes Rpo26, a subunit common to all three yeast nuclear RNA polymerases (RNAP). Mutations in the Abf1p binding site were identified among a pool of rpo26 mutant alleles that confer synthetic lethality in combination with a temperature-sensitive mutation (rpo21-4) in the gene that encodes the largest subunit of RNAPII (Rpo21p). In the presence of the wild-type allele of RPO21 these rpo26 promoter mutations confer a cold-sensitive growth defect. Electrophoretic mobility-shift assays using purified Abf1p demonstrated that Abf1p binds to the RPO26 promoter and that the promoter mutations abolish this binding in vitro. Quantitation of the amount of RPO26 mRNA showed that mutations in the Abf1p binding site reduce the expression of RPO26 by approximately 60%. Mutations that affect Abf1p binding also result in a shift of the RPO26 transcriptional start sites to positions further upstream than normal. These results suggest that binding of the Abf1p transcription factor to the RPO26 promoter is important not only in establishing the level of transcription for this gene, but also in positioning the initiation sites of transcription.

Underproduction of the largest subunit of RNA polymerase II causes temperature sensitivity, slow growth, and inositol auxotrophy in Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae, mutations in genes encoding subunits of RNA polymerase II (RNAPII) often give rise to a set of pleiotropic phenotypes that includes temperature sensitivity, slow growth and inositol auxotrophy. In this study, we show that these phenotypes can be brought about by a reduction in the intracellular concentration of RNAPII. Underproduction of RNAPII was achieved by expressing the gene (RPO21), encoding the largest subunit of the enzyme, from the LEU2 promoter or a weaker derivative of it, two promoters that can be repressed by the addition of leucine to the growth medium. We found that cells that underproduced RPO21 were unable to derepress fully the expression of a reporter gene under the control of the INO1 UAS. Our results indicate that temperature sensitivity, slow growth and inositol auxotrophy is a set of phenotypes that can be caused by lowering the steady-state amount of RNAPII; these results also lead to the prediction that some of the previously identified RNAPII mutations that confer this same set of phenotypes affect the assembly/stability of the enzyme. We propose a model to explain the hypersensitivity of INO1 transcription to mutations that affect components of the RNAPII transcriptional machinery.

The fission yeast Schizosaccharomyces pombe rpb6 gene encodes the common phosphorylated subunit of RNA polymerase and complements a mutation in the corresponding gene of Saccharomyces cerevisiae

A single-copy gene, homologous to the RPB6 gene from Saccharomyces cerevisiae, encoding a small phosphorylated subunit common to all three forms of nuclear DNA-dependent RNA polymerase was isolated from the fission yeast Schizosaccharomyces pombe. Its cDNA copy consists of an open reading frame of 142 codons and encodes an acidic protein (predicted pI 4.1) with a M(r) of 15,730. The genomic copy of Sz. pombe rpb6 contains an intron (219 nucleotides) located at codon 92, a position which does not correspond to the single intron of the S. cerevisiae gene. The sequencing of both genomic and cDNA copies of rpb6 allowed us to determine the probable positions of the start and stop of rpb6 transcription and to identify a putative TATA box. The primary structures of the Sz. pombe and S. cerevisiae Rpb6 proteins have 60.7% identity, with the same general organization: a highly acidic N-terminal region followed by a short basic region and a C terminus featuring a putative heptad Leu repeat. The C-terminal half of the sequence is particularly well conserved and, therefore, probably contains the most important functional domain. Moreover, a heterospecific complementation test showed that rpb6 from Sz. pombe fully complements a complete deletion of its S. cerevisiae homologue.

An impaired RNA polymerase II activity in Saccharomyces cerevisiae causes cell-cycle inhibition at START

Saccharomyces cerevisiae cells harboring the temperature-sensitive mutation rpo21-4, in the gene encoding the largest subunit of RNA polymerase II, were shown to be partially impaired for cell-cycle progress at a permissive temperature, and to become permanently blocked at the cell-cycle regulatory step, START, at a restrictive temperature. The rpo21-4 mutation was lethal in combination with cdc28 mutations in the p34 protein kinase gene required for START. Transcripts of the CLN1 and CLN2 genes, encoding G1-cyclin proteins that, along with p34, are necessary for START, were decreased in abundance by the rpo21-4 mutation at a restrictive temperature. Increased G1-cyclin production, by expression of the CLN1 or CLN2 genes from a heterologous GAL promoter, overcame the rpo21-4-mediated START inhibition, but such mutant cells nevertheless remained unable to proliferate at a restrictive temperature. These findings reveal that START can be particularly sensitive to an impaired RNA polymerase II function, presumably through effects on G1-cyclin expression.

A general suppressor of RNA polymerase I, II and III mutations in Saccharomyces cerevisiae

A multicopy genomic library of Saccharomyces cerevisiae (strain FL100) was screened for its ability to suppress conditionally defective mutations altering the 31 kDa subunit (rpc31-236) or the 53 kDa subunit (rpc53-254/424) of RNA polymerase III. In addition to allele-specific suppressors, we identified seven suppressor clones that acted on both mutations and also suppressed several other conditional mutations defective in RNA polymerases I or II. All these clones harbored a complete copy of the SSD1 gene. The same pleiotropic suppression pattern was found with the dominant SSD1-v allele present in some laboratory strains of S. cerevisiae. SSD1-v was previously shown to suppress mutations defective in the SIT4 gene product (a predicted protein phosphatase subunit) or in the regulatory subunit of the cyclic AMP-dependent protein kinase. We propose that the SSD1 gene product modulates the activity (or the level) of the three nuclear RNA polymerases, possibly by altering their degree of phosphorylation.

A point mutation in the core subunit gene of yeast mitochondrial RNA polymerase is suppressed by a high level of specificity factor MTF1

The temperature-sensitive yeast mutant pet-ts798 is characterized by an altered mitochondrial transcription apparatus. The mutation has previously been shown to map in the RPO41 gene encoding the core enzyme of mitochondrial RNA polymerase. In the present study the rpo41/pet-ts798 allele was cloned and sequenced, demonstrating that the mutant phenotype is caused by a single amino acid change in a conserved region of the core polymerase. The nuclear gene MTF1, previously isolated as a high copy suppressor of mutant rpo41/pet-ts798, and its gene product were characterized in more detail. Import of a MTF1-COXIV fusion protein in vivo and also import studies with in vitro synthesized MTF1 precursors indicate that MTF1 is a mitochondrial protein and that no apparent cleavage occurs during its import into mitochondria. DNA-binding assays demonstrate that the MTF1 protein alone interacts with DNA in a non-specific manner. An antibody directed against specificity factor MTF1 was raised and used for immunological quantification experiments. The results indicate that suppression is mediated by an increased level of MTF1 protein in mitochondria of the rpo41/pet-ts798 mutant. Possible implications of this finding for the mechanism of suppression are discussed.

New twists in class III transcription

Recent work emphasizes the similarity between polymerase II and III in the mechanisms of transcription. Highlights of the past year include the alignment of individual polypeptides within class III transcription complexes and the demonstration that class III transcription machinery includes TFIID and an RNA component.

Isolation and phenotypic analysis of conditional-lethal, linker-insertion mutations in the gene encoding the largest subunit of RNA polymerase II in Saccharomyces cerevisiae

Linker-insertion mutagenesis was used to isolate mutations in the Saccharomyces cerevisiae gene encoding the largest subunit of RNA polymerase II (RPO21, also called RPB1). The mutant rpo21 alleles carried on a plamid were introduced into a haploid yeast strain that conditionally expresses RPO21 from the inducible promoter pGAL10. Growth of this strain on medium containing glucose is sustained only if the plasmid-borne rpo21 allele encodes a functional protein. Of nineteen linker-insertion alleles tested, five (rpo21-4 to -8) were found that impose a temperature-sensitive (ts) lethal phenotype on yeast cells. Four of these five ts alleles encode mutant proteins in which the site of insertion lies near one of the regions of the largest subunit that have been conserved during evolution. Two of the ts mutants (rpo21-4 and rpo21-7) display pleiotropic phenotypes, including an auxotrophy for inositol and a decreased proliferation rate at the permissive temperature. The functional relationship between RPO21 and RPO26, the gene encoding the 17.9 kDa subunit shared by RNA polymerases I, II, and III was investigated by determining the ability of increased dosage of RPO26 to suppress the ts phenotype imposed by rpo21-4 to -8. Suppression of the ts defect was specific for the rpo21-4 allele and was accompanied by co-suppression of the inositol auxotrophy. These results suggest that mutations in the largest subunit of RNA polymerase II can have profound effects on the expression of specific subsets of genes, such as those involved in the metabolism of inositol. In the rpo21-4 mutant, these pleiotropic phenotypes can be attributed to a defective interaction between the largest subunit and the RPO26 subunit of RNA polymerase II.