Yeast cells respond to a shift to higher osmolarity by increasing the cellular content of the osmolyte glycerol. This response is accompanied by a stimulation of the expression of genes encoding enzymes in the glycerol production pathway. In this study the osmotic induction of one of those genes, which encodes glycerol-3-phosphate dehydrogenase, was monitored in time course experiments. The response is independent of the osmolyte and consists of four apparent phases: a lag phase, an initial induction phase, a feedback phase and a sustained long-term induction. Osmotic shock with progressively higher osmolyte concentrations caused a prolonged lag phase. Deletion of which encodes the terminal protein kinase of the high osmolarity glycerol (HOG) response pathway, led to an even longer lag phase and drastically lower basal and induced mRNA levels. However, the induction was only moderately diminished. Overstimulation of Hog1p by deletion of the genes for the protein phosphatases and led to higher basal and induced 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 expression. Although expression has so far not been reported to be controlled by a general stress response mechanism, heat-shock induction of the mRNA level was observed. However, unregulated protein kinase A activity, which strongly affects the general stress response, only marginally altered the mRNA level of . The osmotic stimulation of expression does not seem to be mediated by derepression, since deletion of the gene, which encodes a general repressor, did not significantly alter the induction profile. A hypo-osmotic shock led to a transient 10-fold drop of the mRNA level. Neither the HOG nor the protein kinase C pathway, which is stimulated by a decrease in external osmolarity, is involved in this effect. It was concluded that osmotic regulation of expression is the result of an interplay between different signalling pathways, some of which remain to be identified.


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