Thompson, Suzanne Claire; (2001) Analysis of the weak acid stress response in Saccharomyces cerevisiae. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Stress response pathways serve a vital role throughout nature. Recently, a discrete weak acid stress response pathway has been identified in the yeast Saccharomyces cerevisiae. Weak acids are commonly used in the food industry to preserve low pH food and beverages. The current understanding is that their mode of action is to reduce intracellular pH (pHi) to an inhibitory level. At low pH these compounds exert the greatest antimicrobial effect. Despite this S. cerevisiae is able to grow at low pH in the presence of weak acid food preservatives, enabling it to spoil manufactured products. Work outlined in this thesis attempts to further the understanding of the response elicited by yeast to overcome preservative inhibition. Exposure to weak acids leads to the induction of the ABC-transporter, Pdr12p. This plays an important role in weak acid adaptation since Δpdr12 cells are hypersensitive to sorbic and benzoic acid. The regulation of Pdr12p was investigated. Results presented in this work indicate that the Cmk1 isoform of Ca2+/Calmodulin dependent protein kinase affects Pdr12p; since Δcmk1 strains are more resistant to weak acids and constitutively pump fluorescein, a known substrate of Pdr12p. The study also aimed to identify the regulation of the PDR12 gene, encoding Pdr12p. A transcription factor important for the control of Pdr12p expression at low pH is identified in this study, via complementation cloning of a weak acid sensitive mutant, as the product of the RLM1 gene. ATP dependent pumping of weak acid anions by Pdr12p is energetically expensive to the cell and indeed futile unless diffusional entry of weak acids into the cell is limited. The MMN5 gene, encoding a mannosyltransferase enzyme involved in the synthesis of the cell wall, was also identified via complementation cloning of weak acid sensitive strains made in this study. This indicates that proper cell wall structure is vital for weak acid resistance, changes to this structure possibly contributing to limitation of weak acid entry to the cell.

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