The InsP7 phosphatase Siw14 regulates inositol pyrophosphate levels to control localization of the general stress response transcription factor Msn2

The environmental stress response (ESR) is critical for cell survival. Yeast cells unable to synthesize inositol pyrophosphates (PP-InsPs) are unable to induce the ESR. We recently discovered a diphosphoinositol pentakisphosphate (PP-InsP₅) phosphatase in Saccharomyces cerevisiae encoded by SIW14 Yeast strains deleted for SIW14 have increased levels of PP-InsPs. We hypothesized that strains with high inositol pyrophosphate levels will have an increased stress response. We examined the response of the siw14Δ mutant to heat shock, nutrient limitation, osmotic stress, and oxidative treatment using cell growth assays and found increased resistance to each. Transcriptional responses to oxidative and osmotic stresses were assessed using microarray and reverse transcriptase quantitative PCR. The ESR was partially induced in the siw14Δ mutant strain, consistent with the increased stress resistance, and the mutant strain further induced the ESR in response to oxidative and osmotic stresses. The levels of PP-InsPs increased in WT cells under oxidative stress but not under hyperosmotic stress, and they were high and unchanging in the mutant. Phosphatase activity of Siw14 was inhibited by oxidation that was reversible. To determine how altered PP-InsP levels affect the ESR, we performed epistasis experiments with mutations in rpd3 and msn2/4 combined with siw14Δ. We show that mutations in msn2Δ and msn4Δ, but not rpd3, are epistatic to siw14Δ by assessing growth under oxidative stress conditions and expression of CTT1 Msn2-GFP nuclear localization was increased in the siw14Δ. These data support a model in which the modulation of PP-InsPs influence the ESR through general stress response transcription factors Msn2/4.

Nutritional and meiotic induction of transiently heritable stress resistant states in budding yeast

Transient exposures to environmental stresses induce altered physiological states in exposed cells that persist after the stresses have been removed. These states, referred to as cellular memory, can even be passed on to daughter cells and may thus be thought of as embodying a form of epigenetic inheritance. We find that meiotically produced spores in the budding yeast S. cerevisiae possess a state of heightened stress resistance that, following their germination, persists for numerous mitotic generations. As yeast meiotic development is essentially a starvation response that a/alpha diploid cells engage, we sought to model this phenomenon by subjecting haploid cells to starvation conditions. We find also that haploid cells exposed to glucose withdrawal acquire a state of elevated stress resistance that persists after the reintroduction of these cells to glucose-replete media. Following release from lengthy durations of glucose starvation, we confirm that this physiological state of enhanced stress resistance is propagated in descendants of the exposed cells through two mitotic divisions before fading from the population. In both haploid starved cells and diploid produced meiotic spores we show that their cellular memories are not attributable to trehalose, a widely regarded stress protectant that accumulates in these cell types. Moreover, the transiently heritable stress resistant state induced by glucose starvation in haploid cells is independent of the Msn2/4 transcription factors, which are known to program cellular memory induced by exposure of cells to NaCl. Our findings identify new developmentally and nutritionally induced states of cellular memory that exhibit striking degrees of persistence and mitotic heritability.