Different signalling pathways contribute to the control of GPD1 gene expression by osmotic stress in Saccharomyces cerevisiae

Yeast cells respond to a shift to higher osmolarity by increasing the cellu lar content of the osmolyte glycerol. This response is accompanied by a sti mulation of the expression of genes encoding enzymes in the glycerol produc tion pathway. In this study the osmotic induction of one of those genes, GP D1, which encodes glycerol-3-phosphate dehydrogenase, was monitored in time course experiments. The response is independent of the osmolyte and consis ts of four apparent phases: a lag phase, an initial induction phase, a feed back phase and a sustained long-term induction. Osmotic shock with progress ively higher osmolyte concentrations caused a prolonged lag phase. Deletion of HOG1, which encodes the terminal protein kinase of the high osmolarity glycerol (HOG) response pathway, led to an even longer lag phase and drasti cally lower basal and induced GPD1 mRNA levels. However, the induction was only moderately diminished. Overstimulation of Hog1p by deletion of the gen es for the protein phosphatases PTP2 and PTP3 led to higher basal and induc ed mRNA levels and a shorter lag phase. The protein phosphatase calcineurin . which mediates salt-induced expression of some genes, does not appear to contribute to the control of GPD1 expression. Although GPD1 expression has so far not been reported to be controlled by a general stress response mech anism, heat-shock induction of the GPD1 mRNA level was observed. However, u nregulated protein kinase A activity, which strongly affects the general st ress response, only marginally altered the mRNA level of GPD1. The osmotic stimulation of GPD1 expression does not seem to be mediated by derepression , since deletion of the SSN6 gene, which encodes a general repressor, did n ot significantly alter the induction profile. A hypoosmotic shock led to a transient 10-fold drop of the GPD1 mRNA level. Neither the HOG nor the prot ein kinase C pathway, which is stimulated by a decrease in external osmolar ity, is involved in this effect. It was concluded that osmotic regulation o f GPD1 expression is the result of an interplay between different signallin g pathways, some of which remain to be identified.