Relative levels of RNA polII subunits differentially affect starvation response in budding yeast.

The Rpb4/7 subcomplex of RNA polymerase II in Saccharomyces cerevisiae is known to play an important role in stress response and stress survival. These two proteins perform overlapping functions ensuring an appropriate cellular response through transcriptional regulation of gene expression. Rpb4 and Rpb7 also perform many cellular functions either together or independent of one another. Here, we show that Rpb4 and Rpb7 differently affect during the nutritional starvation response pathways of sporulation and pseudohyphae formation. Rpb4 enhances the cells' proficiency to sporulate but suppresses pseudohyphal growth. On the other hand, Rpb7 promotes pseudohyphal growth and suppresses sporulation in a dose-dependent manner. We present a model whereby the stoichiometry of Rpb4 and Rpb7 and their relative levels in the cell play a switch like role in establishing either sporulation or pseudohyphal gene expression.

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.