Chromatin conformations of HSP12 during transcriptional activation in the Saccharomyces cerevisiae stationary phase.

During evolution, living cells have developed sophisticated molecular and physiological processes to cope with a variety of stressors. These mechanisms, which collectively constitute the Environmental Stress Response, involve the activation/repression of hundreds of genes that are regulated to respond rapidly and effectively to protect the cell. The main stressors include sudden increases in environmental temperature and osmolarity, exposure to heavy metals, nutrient limitation, ROS accumulation, and protein-damaging events. The growth stages of the yeast S. cerevisiae proceed from the exponential to the diauxic phase, finally reaching the stationary phase. It is in this latter phase that the main stressor events are more active. In the present work, we aim to understand whether the responses evoked by the sudden onset of a stressor, like what happens to cells going through the stationary phase, would be different or similar to those induced by a gradual increase in the same stimulus. To this aim, we studied the expression of the HSP12 gene of the HSP family of proteins, typically induced by stress conditions, with a focus on the role of chromatin in this regulation. Analyses of nucleosome occupancy and three-dimensional chromatin conformation suggest the activation of a different response pathway upon a sudden vs a gradual onset of a stress stimulus. Here we show that it is the three-dimensional chromatin structure of HSP12, rather than nucleosome remodeling, that becomes altered in HSP12 transcription during the stationary phase.