Mechanisms of transcriptional induction in yeast

The Ras pathway is involved in multiple signaling pathways. I demonstrate that deregulation of protein kinase A (PKA), the effector of Ras in yeast, results in a significant increase in ribosomal protein gene (RPG) transcription. Previously defined growth rate control of ribosome synthesis results primarily from the coordinate regulation of ribosomal protein gene (RPG) transcription. I now show that strains which are unable to regulate PKA activity fail to mediate either of the two known RPG responses to growth signals. I have found that a RAP1 binding site is necessary and sufficient to confer the PKA-dependent transcriptional increase seen for RPGs. These results suggest that modulation of RAP1 transcriptional activity by PKA accounts for growth-regulated expression of RPGs.;The rate at which the TATA-binding protein (TBP) interacts with the TATA element and promotes transcription by RNA polymerase II was determined in yeast cells. A TBP derivative with altered TATA-element specificity was rapidly induced, and transcription from promoters with appropriately mutated TATA elements was measured. Without a functional activator protein, basal transcription was observed only after a lag of several hours. In contrast, GCN4-activated transcription occurred rapidly upon induction of the TBP derivative. These results suggest that accessibility of TBP to the chromatin template in vivo is rate-limiting and that activation domains increase recruitment of TBP to the promoter.;In collaboration with Michael Karin's lab at UCSD we have defined signals in yeast that are not associated with growth but which require the induction of the Ras pathway. We have found that there is a DNA damage independent response in S. cerevisiae mediated by the yeast AP-1 factor, Gcn4, and the Ras pathway, which is conserved from yeast to mammals. This pathway results from a mechanistically novel translational induction as well a post-translational event. This response serves a protective function for the cell, conferring a fifteen-fold decreased sensitivity to UV-irradiation. We have also shown that a previously defined transient increase in Gcn4 levels seen when yeast strains are transferred from rich to amino-acid starvation medium is mediated by the Ras pathway.