Weak acid preservatives block the heat shock response and heat-shock-element-directed lacZ expression of low pH Saccharomyces cerevisiae cultures, an inhibitory action partially relieved by respiratory deficiency.

Inhibition of microbial growth by weak acid preservatives increases with medium acidification, since these agents enter cells in the undissociated state. Many of the effects of these acids are due to the cytoplasmic acidification they cause as they dissociate in the higher pH environment of the cytosol. Sorbic and benzoic acids, two widely used preservatives, were found to exert pronounced effects on the heat shock response and thermotolerance of Saccharomyces cerevisiae. These effects were strongly influenced by the pH of the culture medium. In low pH cultures sorbate inhibited the induction of thermotolerance by sublethal heat shock, causing strong induction of respiratory-deficient petites among the survivors of heat treatment. However, when the culture pH was above 5.5 sorbate acted as a powerful chemical inducer of thermotolerance in the absence of any sublethal heat treatment. Sorbate and benzoate also inhibited heat induction of the major heat shock proteins in low pH yeast cultures. This appears to result from lack of induction of the heat shock element (HSE) promoter sequence since sorbate prevented heat induction of a HSE-lacZ fusion at low pH. The uncoupler carbonyl cyanide m-chlorophenylhydrazone (CCCP) and the plasma-membrane-ATPase inhibitor diethylstilboestrol were identified as additional inhibitors of heat induction of heat shock proteins. Numerous chemicals induce the heat shock response in the absence of heat stress, but sorbate, benzoate, CCCP and diethylstilboestrol are the first compounds shown to act as selective inhibitors of heat-induced protein expression in yeast. In the presence of sorbate concentrations which, at low pH, totally inhibit both the heat shock response and growth of cells competent in respiration, respiratory-deficient petites still retain a limited capacity for growth and for heat induction of heat shock proteins. This restoration of a response to heat shock in acidified sorbate-treated cultures of petites might contribute to their higher capacity for growth in the presence of sorbate.