Single-molecule mRNA decay measurements reveal promoter- regulated mRNA stability in yeast.

Messenger RNA decay measurements are typically performed on a population of cells. However, this approach cannot reveal sufficient complexity to provide information on mechanisms that may regulate mRNA degradation, possibly on short timescales. To address this deficiency, we measured cell cycle-regulated decay in single yeast cells using single-molecule FISH. We found that two genes responsible for mitotic progression, SWI5 and CLB2, exhibit a mitosis-dependent mRNA stability switch. Their transcripts are stable until mitosis, when a precipitous decay eliminates the mRNA complement, preventing carryover into the next cycle. Remarkably, the specificity and timing of decay is entirely regulated by their promoter, independent of specific cis mRNA sequences. The mitotic exit network protein Dbf2p binds to SWI5 and CLB2 mRNAs cotranscriptionally and regulates their decay. This work reveals the promoter-dependent control of mRNA stability, a regulatory mechanism that could be employed by a variety of mRNAs and organisms.

SF3B1/Hsh155 HEAT motif mutations affect interaction with the spliceosomal ATPase Prp5, resulting in altered branch site selectivity in pre-mRNA splicing.

Mutations in the U2 snRNP component SF3B1 are prominent in myelodysplastic syndromes (MDSs) and other cancers and have been shown recently to alter branch site (BS) or 3' splice site selection in splicing. However, the molecular mechanism of altered splicing is not known. We show here that hsh155 mutant alleles in Saccharomyces cerevisiae, counterparts of SF3B1 mutations frequently found in cancers, specifically change splicing of suboptimal BS pre-mRNA substrates. We found that Hsh155p interacts directly with Prp5p, the first ATPase that acts during spliceosome assembly, and localized the interacting regions to HEAT (Huntingtin, EF3, PP2A, and TOR1) motifs in SF3B1 associated with disease mutations. Furthermore, we show that mutations in these motifs from both human disease and yeast genetic screens alter the physical interaction with Prp5p, alter branch region specification, and phenocopy mutations in Prp5p. These and other data demonstrate that mutations in Hsh155p and Prp5p alter splicing because they change the direct physical interaction between Hsh155p and Prp5p. This altered physical interaction results in altered loading (i.e., "fidelity") of the BS-U2 duplex into the SF3B complex during prespliceosome formation. These results provide a mechanistic framework to explain the consequences of intron recognition and splicing of SF3B1 mutations found in disease.

Prp5-Spt8/Spt3 interaction mediates a reciprocal coupling between splicing and transcription.

Transcription and pre-mRNA splicing are coupled to promote gene expression and regulation. However, mechanisms by which transcription and splicing influence each other are still under investigation. The ATPase Prp5p is required for pre-spliceosome assembly and splicing proofreading at the branch-point region. From an open UV mutagenesis screen for genetic suppressors of prp5 defects and subsequent targeted testing, we identify components of the TBP-binding module of the Spt-Ada-Gcn5 Acetyltransferase (SAGA) complex, Spt8p and Spt3p. Spt8Δ and spt3Δ rescue the cold-sensitivity of prp5-GAR allele, and prp5 mutants restore growth of spt8Δ and spt3Δ strains on 6-azauracil. By chromatin immunoprecipitation (ChIP), we find that prp5 alleles decrease recruitment of RNA polymerase II (Pol II) to an intron-containing gene, which is rescued by spt8Δ. Further ChIP-seq reveals that global effects on Pol II-binding are mutually rescued by prp5-GAR and spt8Δ. Inhibited splicing caused by prp5-GAR is also restored by spt8Δ. In vitro assays indicate that Prp5p directly interacts with Spt8p, but not Spt3p. We demonstrate that Prp5p's splicing proofreading is modulated by Spt8p and Spt3p. Therefore, this study reveals that interactions between the TBP-binding module of SAGA and the spliceosomal ATPase Prp5p mediate a balance between transcription initiation/elongation and pre-spliceosome assembly.

Pseudouridines in U2 snRNA stimulate the ATPase activity of Prp5 during spliceosome assembly.

Pseudouridine (Ψ) is the most abundant internal modification identified in RNA, and yet little is understood of its effects on downstream reactions. Yeast U2 snRNA contains three conserved Ψs (Ψ35, Ψ42, and Ψ44) in the branch site recognition region (BSRR), which base pairs with the pre-mRNA branch site during splicing. Here, we show that blocks to pseudouridylation at these positions reduce the efficiency of pre-mRNA splicing, leading to growth-deficient phenotypes. Restoration of pseudouridylation at these positions using designer snoRNAs results in near complete rescue of splicing and cell growth. These Ψs interact genetically with Prp5, an RNA-dependent ATPase involved in monitoring the U2 BSRR-branch site base-pairing interaction. Biochemical analysis indicates that Prp5 has reduced affinity for U2 snRNA that lacks Ψ42 and Ψ44 and that Prp5 ATPase activity is reduced when stimulated by U2 lacking Ψ42 or Ψ44 relative to wild type, resulting in inefficient spliceosome assembly. Furthermore, in vivo DMS probing analysis reveals that pseudouridylated U2, compared to U2 lacking Ψ42 and Ψ44, adopts a slightly different structure in the branch site recognition region. Taken together, our results indicate that the Ψs in U2 snRNA contribute to pre-mRNA splicing by directly altering the binding/ATPase activity of Prp5.

Insights into branch nucleophile positioning and activation from an orthogonal pre-mRNA splicing system in yeast.

The duplex formed between the branch site (BS) of a spliceosomal intron and its cognate sequence in U2 snRNA is important for spliceosome assembly and the first catalytic step of splicing. We describe the development of an orthogonal BS-U2 system in S. cerevisiae in which spliceosomes containing a grossly substituted second-copy U2 snRNA mediate the in vivo splicing of a single reporter transcript carrying a cognate substitution. Systematic use of this approach to investigate requirements for branching catalysis reveals considerable flexibility in the sequence of the BS-U2 duplex and its positioning relative to the catalytic center. Branching efficiency depends on the identity of the branch nucleotide, its position within the BS-U2 duplex, and its distance from U2/U6 helix Ia. These results provide insights into substrate selection during spliceosomal branching catalysis; additionally, this system provides a foundation and tool for future mechanistic splicing research.

Endogenous U2·U5·U6 snRNA complexes in S. pombe are intron lariat spliceosomes.

Excision of introns from pre-mRNAs is mediated by the spliceosome, a multi-megadalton complex consisting of U1, U2, U4/U6, and U5 snRNPs plus scores of associated proteins. Spliceosome assembly and disassembly are highly dynamic processes involving multiple stable intermediates. In this study, we utilized a split TAP-tag approach for large-scale purification of an abundant endogenous U2·U5·U6 complex from Schizosaccharomyces pombe. RNAseq revealed this complex to largely contain excised introns, indicating that it is primarily ILS (intron lariat spliceosome) complexes. These endogenous ILS complexes are remarkably resistant to both high-salt and nuclease digestion. Mass spectrometry analysis identified 68, 45, and 43 proteins in low-salt-, high-salt-, and micrococcal nuclease-treated preps, respectively. The protein content of a S. pombe ILS complex strongly resembles that previously reported for human spliced product (P) and Saccharomyces cerevisiae ILS complexes assembled on single pre-mRNAs in vitro. However, the ATP-dependent RNA helicase Brr2 was either substoichiometric in low-salt preps or completely absent from high-salt and MNase preps. Because Brr2 facilitates spliceosome disassembly, its relative absence may explain why the ILS complex accumulates logarithmically growing cultures and the inability of S. pombe extracts to support in vitro splicing.

"Nought may endure but mutability": spliceosome dynamics and the regulation of splicing.

The spliceosome is both compositionally and conformationally dynamic. Each transition along the splicing pathway presents an opportunity for progression, pausing, or discard, allowing splice site choice to be regulated throughout both the assembly and catalytic phases of the reaction.

Splicing proofreading at 5' splice sites by ATPase Prp28p.

Fidelity and efficiency of pre-mRNA splicing are critical for generating functional mRNAs, but how such accuracy in 5' splice site (SS) selection is attained is not fully clear. Through a series of yeast genetic screens, we isolated alleles of prp28 that improve splicing of suboptimal 5'SS substrates, demonstrating that WT-Prp28p proofreads, and consequently rejects, poor 5'SS. Prp28p is thought to facilitate the disruption of 5'SS-U1 snRNA pairing to allow for 5'SS-U6 snRNA pairing in the catalytic spliceosome; unexpectedly, 5'SS proofreading by Prp28p is dependent on competition with the stability of the 5'SS:U6 duplex, but not the 5'SS:U1 duplex. E404K, the strongest prp28 allele containing a mutation located in the linker region between adenosine triphosphatase (ATPase) subdomains, exhibited lower RNA-binding activity and enhanced splicing of suboptimal substrates before first-step catalysis, suggesting that decreased Prp28p activity allows longer time for suboptimal 5'SS substrates to pair with U6 snRNA and thereby reduces splicing fidelity. Residue E404 is critical for providing high splicing activity, demonstrated here in both yeast and Drosophila cells. Thus, the subdomain linker in Prp28p plays important roles both in splicing efficiency across species and in proofreading of 5'SS.

CEF1/CDC5 alleles modulate transitions between catalytic conformations of the spliceosome.

Conformational change within the spliceosome is required between the first and second catalytic steps of pre-mRNA splicing. A prior genetic screen for suppressors of an intron mutant that stalls between the two steps yielded both prp8 and non-prp8 alleles that suppressed second-step splicing defects. We have now identified the strongest non-prp8 suppressors as alleles of the NTC (Prp19 complex) component, CEF1. These cef1 alleles generally suppress second-step defects caused by a variety of intron mutations, mutations in U6 snRNA, or deletion of the second-step protein factor Prp17, and they can activate alternative 3' splice sites. Genetic and functional interactions between cef1 and prp8 alleles suggest that they modulate the same event(s) in the first-to-second-step transition, most likely by stabilization of the second-step spliceosome; in contrast, alleles of U6 snRNA that also alter this transition modulate a distinct event, most likely by stabilization of the first-step spliceosome. These results implicate a myb-like domain of Cef1/CDC5 in interactions that modulate conformational states of the spliceosome and suggest that alteration of these events affects splice site use, resulting in alternative splicing-like patterns in yeast.

Opposing classes of prp8 alleles modulate the transition between the catalytic steps of pre-mRNA splicing.

The spliceosome is thought to undergo a conformational change between the two catalytic steps of precursor messenger RNA splicing, although the specific events in this transition are poorly understood. We previously proposed a two-state model of splicing in which the conformations required for the first and second steps are in competition. Here, we identify and characterize a class of prp8 mutants that suppress first-step splicing defects and oppose the action of the previously described prp8 suppressors of second-step defects; these opposing effects parallel those of ribosomal 'ram' and 'restrictive' mutants, which alter fidelity of transfer RNA decoding. On the basis of genetic interactions, we propose that prp8-mediated substrate repositioning during the transition occurs between catalytic-center opening and closure mediated by the U6 small nuclear RNA and the DExH/D ATPase gene prp16. Modulation of these events alters splice-site selection and splicing fidelity.

Type I and II PRMTs inversely regulate post-transcriptional intron detention through Sm and CHTOP methylation.

Protein arginine methyltransferases (PRMTs) are required for the regulation of RNA processing factors. Type I PRMT enzymes catalyze mono- and asymmetric dimethylation; Type II enzymes catalyze mono- and symmetric dimethylation. To understand the specific mechanisms of PRMT activity in splicing regulation, we inhibited Type I and II PRMTs and probed their transcriptomic consequences. Using the newly developed Splicing Kinetics and Transcript Elongation Rates by Sequencing (SKaTER-seq) method, analysis of co-transcriptional splicing demonstrated that PRMT inhibition resulted in altered splicing rates. Surprisingly, co-transcriptional splicing kinetics did not correlate with final changes in splicing of polyadenylated RNA. This was particularly true for retained introns (RI). By using actinomycin D to inhibit ongoing transcription, we determined that PRMTs post-transcriptionally regulate RI. Subsequent proteomic analysis of both PRMT-inhibited chromatin and chromatin-associated polyadenylated RNA identified altered binding of many proteins, including the Type I substrate, CHTOP, and the Type II substrate, SmB. Targeted mutagenesis of all methylarginine sites in SmD3, SmB, and SmD1 recapitulated splicing changes seen with Type II PRMT inhibition, without disrupting snRNP assembly. Similarly, mutagenesis of all methylarginine sites in CHTOP recapitulated the splicing changes seen with Type I PRMT inhibition. Examination of subcellular fractions further revealed that RI were enriched in the nucleoplasm and chromatin. Taken together, these data demonstrate that, through Sm and CHTOP arginine methylation, PRMTs regulate the post-transcriptional processing of nuclear, detained introns.

Immunopurified small nucleolar ribonucleoprotein particles pseudouridylate rRNA independently of their association with phosphorylated Nopp140.

The isomerization of up to 100 uridines to pseudouridines (Psis) in eukaryotic rRNA is guided by a similar number of box H/ACA small nucleolar RNAs (snoRNAs), each forming a unique small nucleolar ribonucleoprotein particle (snoRNP) with the same four core proteins, NAP57 (also known as dyskerin or Cbf5p), GAR1, NHP2, and NOP10. Additionally, the nucleolar and Cajal body protein Nopp140 (Srp40p) associates with the snoRNPs. To understand the role of these factors in pseudouridylation, we established an in vitro assay system. Short site-specifically (32)P-labeled rRNA substrates were incubated with subcellular fractions, and the conversion of uridine to Psi was monitored by thin-layer chromatography after digestion to single nucleotides. Immunopurified box H/ACA core particles were sufficient for the reaction. SnoRNPs associated quantitatively and reversibly with Nopp140. However, pseudouridylation activity was independent of Nopp140, consistent with a chaperoning role for this highly phosphorylated protein. Although up to 14 bp between the snoRNA and rRNA were required for the in vitro reaction, rRNA pseudouridylation and release occurred in the absence of ATP and magnesium. These data suggest that substrate release takes place without RNA helicase activity but may be aided by the snoRNP core proteins.

A glimpse of the catalytic core of a group II intron.

A paper in a recent issue of Science describes the first high-resolution structure of part of the catalytic core of a group II intron that will allow more detailed comparisons between the excision of introns by self-splicing group II introns and by nuclear pre-mRNA introns.

A pseudouridine residue in the spliceosome core is part of the filamentous growth program in yeast.

Although pseudouridine nucleobases are abundant in tRNAs, rRNAs, and small nuclear RNAs (snRNAs), they are not known to have physiologic roles in cell differentiation. We have identified a pseudouridine residue (Ψ28) on spliceosomal U6 snRNA that is induced during filamentous growth of Saccharomyces cerevisiae. Pus1p catalyzes this modification and is upregulated during filamentation. Several U6 snRNA mutants are strongly pseudouridylated at Ψ28. Remarkably, these U6 mutants activate pseudohyphal growth, dependent upon Pus1p, arguing that U6-Ψ28 per se can initiate at least part of the filamentous growth program. We confirmed this by using a designer small nucleolar RNA (snoRNA) targeting U6-U28 pseudouridylation. Conversely, mutants that block U6-U28 pseudouridylation inhibit pseudohyphal growth. U6-U28 pseudouridylation changes the splicing efficiency of suboptimal introns; thus, Pus1p-dependent pseudouridylation of U6 snRNA contributes to the filamentation growth program.

A U1-U2 snRNP interaction network during intron definition.

The assembly of prespliceosomes is responsible for selection of intron sites for splicing. U1 and U2 snRNPs recognize 5' splice sites and branch sites, respectively; although there is information regarding the composition of these complexes, little is known about interaction among the components or between the two snRNPs. Here we describe the protein network of interactions linking U1 and U2 snRNPs with the ATPase Prp5, important for branch site recognition and fidelity during the first steps of the reaction, using fission yeast Schizosaccharomyces pombe. The U1 snRNP core protein U1A binds to a novel SR-like protein, Rsd1, which has homologs implicated in transcription. Rsd1 also contacts S. pombe Prp5 (SpPrp5), mediated by SR-like domains in both proteins. SpPrp5 then contacts U2 snRNP through SF3b, mediated by a conserved DPLD motif in Prp5. We show that mutations in this motif have consequences not only in vitro (defects in prespliceosome formation) but also in vivo, yielding intron retention and exon skipping defects in fission yeast and altered intron recognition in budding yeast Saccharomyces cerevisiae, indicating that the U1-U2 network provides critical, evolutionarily conserved contacts during intron definition.