Regulation of the immediate growth response in Saccharomyces cerevisiae cultures

An important transition in the life cycle of Saccharomyces cerevisiae occurs when quiescent cultures encounter a fresh batch of nutrients. Within minutes of nutrient encounter S. cerevisiae cells mount a massive transcriptional response. Part of this response, termed Immediate Growth Response (IGR), is characterized by induction of a sixth of the entire S. cerevisiae genome; this group is termed the induced IGR genes. Genes involved in ribosome biogenesis comprise a significant portion of this gene set. Two motifs, RRPE and PAC, are overrepresented at promoters of ribosome biogenesis genes. Both motifs confer activity upon quiescence recovery.;We report that, a previously uncharacterized protein, Stb3 binds at RRPE motifs in vitro and in vivo and regulates expression of a subset of ribosome biogenesis genes. Phenotypically, overexpression of Stb3 slows cellular growth by repressing a group of ribosome biogenesis genes. On the other hand deletion of STB3, while not exhibiting a measurable phenotype, moderately relieves repression from a subset of growth-induced genes during quiescence. These data is consistent with a repressive role of Stb3 during quiescence. Morover Stb3 was found to localize in the nucleus during quiescence.;The transition of S. cerevisiae cells from quiescence to growth involves activity of PKA, TOR and Sch9 growth kinases that convey almost all glucose signaling to express growth genes upon nutrient presence. Our data show that disruption of these growth kinases exacerbated the phenotype of Stb3 while overexpression of SCH9 rescued the phenotype. Furthermore Stb3 relied on presence of Hos2 histone deacetylase to diminish growth; deletion of HOS2 abolished the slow growth phenotype. Histone deacteylation is a common and effective way of gene repression during quiescence and Hos2 is known to affect primarily ribosome biogenesis genes.;These observations taken together point towards a model where Stb3 binds and represses transcription of a subset of ribosomal biogenesis genes during quiescence. This repression could come as a result of Hos2 histone deacetylase that might be recruited at ribosomal biogenesis gene promoters via Stb3.