Measurement of in vivo RNA synthesis rates.

A technique is described to directly measure ongoing transcription from individual genes in permeabilized cells of either the budding yeast Saccharomyces cerevisiae or the fission yeast Schizosaccharomyces pombe. Transcription run-on (TRO) analysis is used to compare the relative rates of synthesis for specific transcripts in cells grown under different environmental conditions or harvested at different stages of development. As the amount of an individual RNA species present at any given time is determined by its net rate of synthesis and degradation, an accurate picture of transcription per se can be obtained only by directly measuring de novo synthesis of RNA (if you are interested in RNA degradation, see Method for measuring mRNA decay rate in Saccharomyces cerevisiae). Most techniques employed to measure changes in the relative levels of individual transcripts present under different conditions, including Northern analysis (see Northern blotting), RT-PCR (see Reverse-transcription PCR (RT-PCR)), nuclease protection assays (see Explanatory Chapter: Nuclease Protection Assays), and genome-wide assays, such as microarray analysis and high throughput RNA sequencing, measure changes in the steady-state level of a transcript, which may or may not reflect the actual changes in transcription of the gene. Recent studies carried out in fission yeast have demonstrated that increases in the steady-state level (accumulation) of many individual mRNAs occur without any significant changes in transcription rates (McPheeters et al., 2009), highlighting the important role of regulated RNA stability in determining gene expression programs (Harigaya et al., 2006).

A complex gene regulatory mechanism that operates at the nexus of multiple RNA processing decisions.

Expression of crs1 pre-mRNA, encoding a meiotic cyclin, is blocked in actively growing fission yeast cells by a multifaceted mechanism. The most striking feature is that in vegetative cells, crs1 transcripts are continuously synthesized but are targeted for degradation rather than splicing and polyadenylation. Turnover of crs1 RNA requires the exosome, as do previously described nuclear surveillance and silencing mechanisms, but does not involve a noncanonical poly(A) polymerase. Instead, crs1 transcripts are targeted for destruction by a factor previously implicated in turnover of meiotic RNAs in growing cells. Like exosome mutants, mmi1 mutants splice and polyadenylate vegetative crs1 transcripts. Two regulatory elements are located at the 3' end of the crs1 gene, consistent with the increased accumulation of spliced RNA in polyadenylation factor mutants. This highly integrated regulatory strategy may ensure a rapid response to adverse conditions, thereby guaranteeing survival.

Psi35 in the branch site recognition region of U2 small nuclear RNA is important for pre-mRNA splicing in Saccharomyces cerevisiae.

Pseudouridine 35 (psi35) in the branch site recognition region of yeast U2 small nuclear RNA is absolutely conserved in all eukaryotes examined. Pus7p catalyzes pseudouridylation at position 35 in Saccharomyces cerevisiae U2. The pus7 deletion strain, although viable in rich medium, is growth-disadvantaged under certain conditions. To clarify the function of U2 psi35 in yeast, we used this pus7 deletion strain to screen a collection of mutant U2 small nuclear RNAs, each containing a point mutation near the branch site recognition sequence, for a synthetic growth defect phenotype. The screen identified two U2 mutants, one containing a U40 --> G40 substitution (U40G) and another having a U40 deletion (U40Delta). Yeast strains carrying either of these U2 mutations grew as well as the wild-type strain in the selection medium, but they exhibited a temperature-sensitive growth defect phenotype when coupled with the pus7 deletion (pus7Delta). A subsequent temperature shift assay and a conditional pus7 depletion (via GAL promoter shutoff) in the U2-U40 mutant genetic background caused pre-mRNA accumulation, suggesting that psi35 is required for pre-mRNA splicing under certain conditions.

Spatial organization of protein-RNA interactions in the branch site-3' splice site region during pre-mRNA splicing in yeast.

A series of efficiently spliced pre-mRNA substrates containing single 4-thiouridine residues were used to monitor RNA-protein interactions involving the branch site-3' splice site-3' exon region during yeast pre-mRNA splicing through cross-linking analysis. Prior to the assembly of the prespliceosome, Mud2p and the branch point bridging protein cross-link to a portion of this region in an ATP-independent fashion. Assembly of the prespliceosome leads to extensive cross-linking of the U2-associated protein Hsh155p to this region. Following the first step of splicing and in a manner independent of Prp16p, the U5 small nuclear ribonucleoprotein particle-associated protein Prp8p also associates extensively with the branch site-3' splice site-3' exon region. The subsequent cross-linking of Prp16p to the lariat intermediate is restricted to the 3' splice site and the adjacent 3' exon sequence. Using modified substrates to either mutationally or chemically block the second step, we found that the association of Prp22p with the lariat intermediate represents an authentic transient intermediate and appears to be restricted to the last eight intron nucleotides. Completion of the second step leads to the cross-linking of an unidentified approximately 80-kDa protein near the branch site sequence, suggesting a potential role for this protein in a later step in intron metabolism. Taken together, these data provide a detailed portrayal of the dynamic associations of proteins with the branch site-3' splice site region during spliceosome assembly and catalysis.