HIGH-OSMOLARITY SIGNALING IN SACCHAROMYCES-CEREVISIAE IS MODULATED INA CARBON-SOURCE-DEPENDENT FASHION

High-osmolarity-induced expression of the small heat-shock gene HSP12 is regulated by the HOG (high-osmolarity glycerol) pathway and PKA (pr otein kinase A). To analyse the regulatory input of both signal transd uction pathways, high-salt-induced HSP12 expression in different genet ic backgrounds on glucose-, ethanol-and glycerol-based culture media w as examined. Upon exposure to high-osmolarity stress, the kinetics of induction of HSP12 in cells growing on the non-fermentable carbon sour ces are strikingly different from those on glucose. Derepression of HS P12 gene expression under non-stress conditions was observed in cells growing on non-fermentable carbon sources. High-salt challenge resulte d in a lower induction of the HSP12 mRNA levels in ethanol-grown cells as compared to glucose-grown cells, whereas in glycerol-grown cells h ardly any high-salt induction of HSP12 mRNA levels could be detected. Analysis of signalling through the HOG pathway suggested that glycerol may influence the activity of this signalling route, possibly via neg ative feedback. Furthermore, the cellular level of PKA activity was fo und to have a great impact on stress-responsive gene transcription. On the basis of the data obtained it was concluded that modulation of PK A activity plays a major role in the stress response. A glucose-depend ent, PKA-regulated cellular component is postulated to affect high-osm olarity-induced HSP12 expression.