Seymour, Ian Jonathan; (1998) Weak acid effects on gene activation in yeast. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Weak lipophilic acids have been commonly used as antifungal agents to preserve low pH foods. In low pH cultures weak acids rapidly enter the cell, dissociating to cause intracellular acidification. The plasma membrane H+-ATPase maintains pH homeostasis by the active extrusion of protons, but this is energetically expensive causing reduced growth rates and biomass yield. All cells show rapid responses when exposed to unfavourable environmental conditions which appear to increase their ability to grow under moderately stressful conditions, or survive conditions of more extreme stress. Yeasts, especially Zygosaccharomyces bailii and to a lesser extent, Saccharomyces cerevisiae, can adapt to growth in weak acids at low pH. In S. cerevisiae, weak acids elicit strong induction of two plasma membrane proteins, Hsp30 and Pdr12 which play a major role in this adaptive response. Discrete gene promoter elements responsive to stress have been identified in S. cerevisiae. Investigations carried out for this thesis were aimed at characterising the effects of weak acids on the expression of stress gene promoters and stress promoter elements. Research described in chapter 3 revealed that weak acids can both inhibit the heat shock activation of stress gene promoters and stress promoter elements in low pH cultures or act as chemical inducers of these same sequences in the absence of heat shock, possibly by a mechanism involving intracellular pH. Studies on the HSP30 promoter and stress-responsive transcription factor mutants (chapter 4) demonstrated that none of the previously characterised stress signalling pathways are involved in the stress-induced activation of this gene. This is proof for the existence of a novel stress response pathway. Chapter 5 outlined a strategy to isolate homologues of S. cerevisiae HSP30 in other yeast species. Although the approach used was unsuccessful Candida albicans and Hansenula polymorpha were shown to contain DNA sequences that hybridized to HSP30 at high stringency.

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