In response to osmotic shock in Saccharomyces cerevisiae the MAP kinase Hog1 coordinates a large-scale transcriptional stress response, rapidly producing hundreds of copies of specified transcripts. Many of the most highly induced genes are bound and regulated by a transcription factor, Sko1, but lack the canonical binding site for this factor. We use ChIP-seq to demonstrate a stress-specific binding mode of Sko1. In stress, Sko1 binds to promoters in close proximity to Hog1, and another Hog1-regulated transcription factor, Hot1. This mode of Sko1 binding requires the physical presence of Hog1, but not Hog1 phosphorylation of Sko1. We identify candidate Sko1 and Hot1 binding motifs that predict co-localization of Sko1, Hot1, and Hog1 at promoters. We then demonstrate a role for Sko1 and Hot1 in directing Hog1-associated RNA Pol II to target genes, where Hog1 is present with the elongating polymerase. We suggest a possible model for Hog1 reprogramming of transcription in the early stages of the osmotic stress response. We then determine the extent and structure of the Hog1 controlled transcriptional program in a related stress, damage to the cell wall. We find that Sko1 and Hot1 have different apparent thresholds for activation by Hog1. In addition, in cell wall damage, Hog1 regulates an additional transcription factor, Rlm1, that is not involved in other Hog1 regulated stress responses. This factor is activated by the coincidence of a signal from Hog1 with that of another MAP kinase, Slt2.

En el llevat Saccharomyces cerevisiae els canvis en les condicions osmòtiques del medi extracel.lular són sensades per la MAP cinasa Hog1, la qual permet dur a terme l'adaptació cel.lular mitjançant la modulació de l'expressió gènica, de la traducció i de la progressió del cicle cel.lular. A l'inici d'aquest projecte de tesi, els mecanismes pels quals Hog1 controla l'expressió gènica no eren del tot coneguts. El nostre objectiu va ser caracteritzar el mecanisme molecular pel qual Hog1 modula la transcripció en resposta a estrès osmòtic. Hem aconseguit demostrar que el reclutament de Hog1 als promotors sensibles a estrès osmòtic per part del factor de transcripció és essencial per al reclutament i activació de la RNA polimerasa II, mecanisme que podria estar conservat en les cèl.lules eucariotes. També hem identificat noves activitats remodeladores de cromatina implicades en la resposta gènica a osmoestrès mediada per Hog1. Vàrem realitzar un cribatge genètic per identificar mutacions que provoquessin osmosensibilitat i una reducció en l'expressió de gens de resposta a estrès osmòtic. Aquest cribatge ens va permetre identificar nous reguladors de la transcripció mediada per osmoestrès: la histona deacetilasa Rpd3 i els complexes SAGA i mediador. Els nostres resultats permeten, doncs, definir un important paper per a Rpd3, SAGA i mediador en la inducció gènica mediada per Hog1, i han estat importants per assolir una millor visió de com les cinases activades per estrès regulen la iniciació de la transcripció. / In Saccharomyces cerevisiae, changes in the extracellular osmotic conditions are sensed by the HOG MAPK pathway, which elicits the program for cell adaptation, including modulation of gene expression, translation and cell-cycle progression. At the beginning of this PhD Project, the mechanisms by which Hog1 was controlling gene transcription were not completely understood. Our main objective was to characterize the molecular mechanisms by which the Hog1 MAPK modulates transcription upon osmostress. We have shown that anchoring of Hog1 to osmoresponsive promoters by the transcription factor is essential for recruitment and activation of RNA polymerase II, a mechanism that might be conserved among eukaryotic cells. In addition, we identified novel chromatin modifying and remodelling activities involved in the Hog1-mediated osmostress gene expression. We performed a genome-wide genetic screening searching for mutations that render cells osmosensitive and displayed reduced expression of osmoresponsive genes. Rpd3 histone deacetylase, SAGA and Mediator complexes were identified as novel regulators of osmostress-mediated transcription. Thus, our results define a major role for Rpd3, SAGA and Mediator in the Hog1-mediated osmostress gene induction, and have been important to achieve a better view of how a SAPK regulates transcription initiation.