Summary auto-generated
This study examined how osmotic stress regulates expression of the GPD1 gene in baker's yeast (Saccharomyces cerevisiae), which encodes an enzyme critical for glycerol production. Researchers monitored GPD1 mRNA levels after subjecting cells to various osmotic stresses using time-course experiments. The response showed four distinct phases: a lag phase, initial rapid induction (20-fold increase within 30 minutes), a feedback decline, and sustained long-term elevation (4-5 fold). Deletion of HOG1, encoding a key protein kinase in the high osmolarity glycerol (HOG) response pathway, prolonged the lag phase and reduced both basal and induced mRNA levels, though induction still occurred. Conversely, deleting phosphatase genes PTP2 and PTP3 (which regulate the HOG pathway) accelerated the response and increased mRNA levels. The calcineurin phosphatase, protein kinase C pathway, and cAMP-dependent protein kinase A showed minimal or no involvement. Heat shock independently induced GPD1 expression, suggesting multiple regulatory mechanisms. Overall, the findings demonstrate that GPD1 osmotic induction results from interplay between the HOG pathway and additional unidentified signaling mechanisms.
Key findings
- GPD1 expression shows a complex four-phase response to osmotic stress, with initial 20-25 fold induction followed by feedback decrease and sustained elevation
- The HOG pathway significantly controls GPD1 expression timing and amplitude; HOG1 deletion delays response and reduces induction, while PTP2/PTP3 deletion accelerates it
- PKA, calcineurin phosphatase, and PKC pathways contribute minimally or not at all to osmotic regulation of GPD1
- Heat shock independently induces GPD1 expression, indicating multiple stress-responsive mechanisms regulate this gene
- GPD1 regulation is osmolyte-independent but severity-dependent, with higher osmolyte concentrations causing progressively delayed responses
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Abstract
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, GPD1, 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 HOG1, 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 GPD1 mRNA levels. However, the induction was only moderately diminished. Overstimulation of Hog1p by deletion of the genes for the protein phosphatases PTP2 and PTP3 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 GPD1 expression. Although GPD1 expression has so far not been reported to be controlled by a general stress response mechanism, heat-shock induction of the GPD1 mRNA level was observed. However, unregulated protein kinase A activity, which strongly affects the general stress 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 not 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 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 GPD1 expression is the result of an interplay between different signalling pathways, some of which remain to be identified.