The Saccharomyces cerevisiae ATP-binding cassette (ABC) transporter Pdr12p effluxes weak acids such as sorbate and benzoate, thus mediating stress adaptation. In this study, we identify a novel transcription factor, War1p, as the regulator of this stress adaptation through transcriptional induction of PDR12. Cells lacking War1p are weak acid hypersensitive, since they fail to induce Pdr12p. The nuclear Zn 2 Cys 6 transcriptional regulator War1p forms homodimers and is rapidly phosphorylated upon sorbate stress. The appearance of phosphorylated War1p isoforms coincides with transcriptional activation of PDR12. Promoter deletion analysis identified a novel cis-acting weak acid response element (WARE) in the PDR12 promoter required for PDR12 induction. War1p recognizes and decorates the WARE both in vitro and in vivo, as demonstrated by band shift assays and in vivo footprinting. Importantly, War1p occupies the WARE in the presence and absence of stress, demonstrating constitutive DNA binding in vivo. Our results suggest that weak acid stress triggers phosphorylation and perhaps activation of War1p. In turn, War1p activation is necessary for the induction of PDR12 through a novel signal transduction event that elicits weak organic acid stress adaptation.

The aquaglyceroprin Fps1 is responsible for glycerol transport in yeast in response to changes in extracellular osmolarity. Control of Fps1 channel activity in response to hyperosmotic shock involves a redundant pair of regulators, Rgc1 (regulator of the glycerol channel 1) and Rgc2, and the MAPK Hog1 (high-osmolarity glycerol response 1). However, the mechanism by which these factors influence channel activity is unknown. We show that Rgc2 maintains Fps1 in the open channel state in the absence of osmotic stress by binding to its C-terminal cytoplasmic domain. This interaction involves a tripartite pleckstrin homology (PH) domain within Rgc2 and a partial PH domain within Fps1. Activation of Hog1 in response to hyperosmotic shock induces the rapid eviction of Rgc2 from Fps1 and consequent channel closure. Hog1 was recruited to the N-terminal cytoplasmic domain of Fps1, which it uses as a platform from which to multiply phosphorylate Rgc2. Thus, these results reveal the mechanism by which Hog1 regulates Fps1 in response to hyperosmotic shock.

SummaryWe determined the genome-wide environmental stress response (ESR) expression profile of Candida glabrata, a human pathogen related to Saccharomyces cerevisiae. Despite different habitats, C. glabrata, S. cerevisiae, Schizosaccharomyces pombe and Candida albicans have a qualitatively similar ESR. We investigate the function of the C. glabrata syntenic orthologues to the ESR transcription factor Msn2. The C. glabrata orthologues CgMsn2 and CgMsn4 contain a motif previously referred to as HD1 (homology domain 1) also present in Msn2 orthologues from fungi closely related to S. cerevisiae. We show that regions including this motif confer stress-regulated intracellular localization when expressed in S. cerevisiae. Site-directed mutagenesis confirms that nuclear export of CgMsn2 in C. glabrata requires an intact HD1. Transcript profiles of CgMsn2/4 mutants and CgMsn2 overexpression strains show that they regulate a part of the CgESR. CgMsn2 complements a S. cerevisiae msn2 null mutant and in stressed C. glabrata cells, rapidly translocates from the cytosol to the nucleus. CgMsn2 is required for full resistance against severe osmotic stress and rapid and full induction of trehalose synthesis genes (TPS1, TPS2). Constitutive activation of CgMsn2 is detrimental for C. glabrata. These results establish an Msn2-regulated general stress response in C. glabrata.