Transcriptional and translational regulation of major heat shock proteins and patterns of trehalose mobilization during hyperthermic recovery in repressed and derepressedSaccharomyces cerevisiae

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Abstract
Patterns of heat shock gene transcription and translation, as well as trehalose content, were investigated in both glucose (repressed) and acetate (derepressed) grown cells of Saccharomyces cerevisiae during heat shock and subsequent return of cells to 25°C. Heat-shocked cells (37°C for 30 min), grown in either glucose- or acetate-supplemented media, initially acquired high thermotolerance to a 50°C heat stress, which was progressively lost when cultures were allowed to recover at 25°C and subsequently exposed to a second heat stress. In all cases, with the notable exception of repressed cells of a relatively thermosensitive strain, inhibition of protein synthesis and coincident decrease in trehalose accumulation during the heat shock had little effect on the kinetics of loss of thermotolerance. Heat shock at 37°C elicited a marked increase in transcription and translation of genes encoding major heat shock proteins (hsps). During recovery at 25°C, both metabolic activities were suppressed followed by a gradual increase in hsp mRNA transcription to levels observed prior to heat shock. De novo translation of hsp mRNAs, however, was no longer observed during the recovery phase, although immuno- detection analyses demonstrated persistence of high levels of hsps 104, 90, 70, and 60 in cells throughout the 240-min recovery period. In addition, while heat shock induced trehalose was rapidly degraded during recovery in repressed cells, levels remained high in derepressed cells. Results therefore indicated that the progressive loss of induced thermotolerance exhibited by glucose- and acetate-grown cells was not closely correlated with levels of hsp or trehalose. It was concluded that both constitutive and de novo synthesized hsps require heat shock associated activation to confer thermotolerance and this modification is progressively reversed upon release from the heat-shocked state.Key words: thermotolerance, hyperthermic recovery, hsp transcription, hsp translation, trehalose.