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

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.

A cis-regulatory mutation in troponin-I of Drosophila reveals the importance of proper stoichiometry of structural proteins during muscle assembly

Rapid and high wing-beat frequencies achieved during insect flight are powered by the indirect flight muscles, the largest group of muscles present in the thorax. Any anomaly during the assembly and/or structural impairment of the indirect flight muscles gives rise to a flightless phenotype. Multiple mutagenesis screens in Drosophila melanogaster for defective flight behavior have led to the isolation and characterization of mutations that have been instrumental in the identification of many proteins and residues that are important for muscle assembly, function, and disease. In this article, we present a molecular-genetic characterization of a flightless mutation, flightless-H (fliH), originally designated as heldup-a (hdp-a). We show that fliH is a cis-regulatory mutation of the wings up A (wupA) gene, which codes for the troponin-I protein, one of the troponin complex proteins, involved in regulation of muscle contraction. The mutation leads to reduced levels of troponin-I transcript and protein. In addition to this, there is also coordinated reduction in transcript and protein levels of other structural protein isoforms that are part of the troponin complex. The altered transcript and protein stoichiometry ultimately culminates in unregulated acto-myosin interactions and a hypercontraction muscle phenotype. Our results shed new insights into the importance of maintaining the stoichiometry of structural proteins during muscle assembly for proper function with implications for the identification of mutations and disease phenotypes in other species, including humans.

Mapping of transcription start sites in Saccharomyces cerevisiae using 5' SAGE

A minimally addressed area in Saccharomyces cerevisiae research is the mapping of transcription start sites (TSS). Mapping of TSS in S.cerevisiae has the potential to contribute to our understanding of gene regulation, transcription, mRNA stability and aspects of RNA biology. Here, we use 5' SAGE to map 5' TSS in S.cerevisiae. Tags identifying the first 15-17 bases of the transcripts are created, ligated to form ditags, amplified, concatemerized and ligated into a vector to create a library. Each clone sequenced from this library identifies 10-20 TSS. We have identified 13,746 unique, unambiguous sequence tags from 2231 S.cerevisiae genes. TSS identified in this study are consistent with published results, with primer extension results described here, and are consistent with expectations based on previous work on transcription initiation. We have aligned the sequence flanking 4637 TSS to identify the consensus sequence A(A(rich))5NPyA(A/T)NN(A(rich))6, which confirms and expands the previous reported PyA(A/T)Pu consensus pattern. The TSS data allowed the identification of a previously unrecognized gene, uncovered errors in previous annotation, and identified potential regulatory RNAs and upstream open reading frames in 5'-untranslated region.