When yeast cells are subjected to environmental changes a rapid and specific reprogramming of the cellular gene expression pattern is required for cell survival. Stress causes a general down-regulation of gene expression together with the induction of a set of stress-responsive genes. The p38-related stress-activated protein kinase Hog1 is an important regulator of osmoresponsive gene expression by controlling critical steps of the mRNA biogenesis.
Mechanistically, upon osmotic stress Hog1 becomes activated and accumulates in the nucleus where binds to the osmoresponsive promoters through specific transcription factors. Once there Hog1 may modulate initiation of transcription by direct phosphorylation of these osmoresponsive transcription factors, by recruiting RNA polymerase II, co-activators and other enzymatic activities such as the Rpd3 histone deacetylase and the Ubp3 deubiquitinase to the osmoresponsive promoters. Hog1 also binds to the coding regions of osmoresponsive genes, acting as a stress-specific transcription elongation factor. Moreover, Hog1 is able to displace nucleosomes by targeting chromatin remodeling activities, such as the RSC complex, to the osmoresponsive genes. Hog1 is also known to be involved in other gene-expression- related processes, such as mRNA export, processing or translation in response to stress.