High-osmolarity-induced expression of the small heat-shock gene is regulated by the HOG (high-osmolarity glycerol) pathway and PKA (protein kinase A). To analyse the regulatory input of both signal transduction pathways, high-salt-induced expression in different genetic backgrounds on glucose-, ethanol- and glycerol-based culture media was examined. Upon exposure to high-osmolarity stress, the kinetics of induction of in cells growing on the non-fermentable carbon sources are strikingly different from those on glucose. Derepression of gene expression under non-stress conditions was observed in cells growing on non-fermentable carbon sources. High-salt challenge resulted in a lower induction of the mRNA levels in ethanol-grown cells as compared to glucose-grown cells, whereas in glycerol-grown cells hardly any high-salt induction of 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 negative feedback. Furthermore, the cellular level of PKA activity was found to have a great impact on stress-responsive gene transcription. On the basis of the data obtained it was concluded that modulation of PKA activity plays a major role in the stress response. A glucose-dependent, PKA-regulated cellular component is postulated to affect high-osmolarity-induced expression.


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