Surprising regulatory plasticity for the conserved HOG pathway in diverse Saccharomyces cerevisiae strains

Timely signaling pathways activation allows cells to survive diverse environmental stress conditions. Mitogen-activated protein kinases (MAPKs) are a highly conserved class of signaling molecules in eukaryotes with essential functions in cellular responses to stress. In Saccharomyces cerevisiae, the role of MAPK Hog1 as a master regulator of the coordinated response to osmotic stress is well understood. However, recent findings suggest that the role of Hog1 may extend beyond canonical osmoadaptation. This study investigates the role of Hog1 in mediating transcriptional responses to acute oxidative and ethanol stress. By harnessing the natural variation present in wild strains of S. cerevisiae, we use gene knockouts, comparative transcriptomics, and survival assays to determine Hog1's involvement in stress responses beyond osmoadaptation. Our findings demonstrate that Hog1 mediates transcriptional reprogramming for non-osmotic stress response in a strain-dependent manner. Osmospecificity of Hog1 activity was identified in the DBY8268 laboratory strain, while differential gene expression was observed in HOG1 knockouts of all wild strains tested under both oxidative and ethanol stress. Further, our data indicate that the function of Hog1 in the response to non-osmotic stress is distinct from the canonical response, with effects ranging from altered ribosomal protein expression dynamics to altered environmental stress response (ESR) activity. Differences in expression correlate with fitness defects of hog1? mutants. These results suggest a generalized role of the Hog1 MAPK in S. cerevisiae, consistent with an evolutionarily generalized function for this kinase, underscoring the importance of genomic diversity for elucidating stress signalling dynamics in yeast. Overall design: Bulk RNA-seq was performed on S. cerevisiae cultures comparing HOG1 wild-type and hog1 deletion mutants in three distinct strains: DBY8268-derivative (Laboratory strain), YPS606 (Oak isolate), and M22 (Vineyard isolate). For each strain and genotype, we collected samples under (i) unstressed conditions, (ii) acute exposure to 0.4 M NaCl, 5% (v/v) ethanol, or 0.4 mM hydrogen peroxide. Stressed samples were collected after 30 (ethanol and peroxide) or 45 minutes (NaCl) of exposure. For each condition, 4 biological replicates were sequenced. One strain was resequenced in triplicate once its original samples were discovered to be following sequencing.