Weak Organic Acid Stress Triggers Hyperphosphorylation of the Yeast Zinc-Finger Transcription Factor War1 and Dampens Stress Adaptation

Activation of Haa1 and War1 transcription factors by differential binding of weak acid anions in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, Haa1 and War1 transcription factors are involved in cellular adaptation against hydrophilic weak acids and lipophilic weak acids, respectively. However, it is unclear how these transcription factors are differentially activated depending on the identity of the weak acid. Using a field-effect transistor (FET)-type biosensor based on carbon nanofibers, in the present study we demonstrate that Haa1 and War1 directly bind to various weak acid anions with different affinities. Haa1 is most sensitive to acetate, followed by lactate, whereas War1 is most sensitive to benzoate, followed by sorbate, reflecting their differential activation during weak acid stresses. We show that DNA binding by Haa1 is induced in the presence of acetic acid and that the N-terminal Zn-binding domain is essential for this activity. Acetate binds to the N-terminal 150-residue region, and the transcriptional activation domain is located between amino acid residues 230 and 483. Our data suggest that acetate binding converts an inactive Haa1 to the active form, which is capable of DNA binding and transcriptional activation.

A Yeast-Based Biosensor for Screening of Short- and Medium-Chain Fatty Acid Production

Short- and medium-chain fatty acids (SMCFA) are important platform chemicals currently produced from nonsustainable resources. The engineering of microbial cells to produce SMCFA, however, lacks high-throughput methods to screen for best performing cells. Here, we present the development of a whole-cell biosensor for easy and rapid detection of SMCFA. The biosensor is based on a multicopy yeast plasmid containing the SMCFA-responsive PDR12 promoter coupled to GFP as the reporter gene. The sensor detected hexanoic, heptanoic and octanoic acid over a linear range up to 2, 1.5, and 0.75 mM, respectively, but did not show a linear response to decanoic and dodecanoic acid. We validated the functionality of the biosensor with culture supernatants of a previously engineered Saccharomyces cerevisiae octanoic acid producer strain and derivatives thereof. The biosensor signal correlated strongly with the octanoic acid concentrations as determined by gas chromatography. Thus, this biosensor enables the high-throughput screening of SMCFA producers and has the potential to drastically speed up the engineering of diverse SMCFA producing cell factories.

Molecular and physiological basis of Saccharomyces cerevisiae tolerance to adverse lignocellulose-based process conditions

Lignocellulose-based biorefineries have been gaining increasing attention to substitute current petroleum-based refineries. Biomass processing requires a pretreatment step to break lignocellulosic biomass recalcitrant structure, which results in the release of a broad range of microbial inhibitors, mainly weak acids, furans, and phenolic compounds. Saccharomyces cerevisiae is the most commonly used organism for ethanol production; however, it can be severely distressed by these lignocellulose-derived inhibitors, in addition to other challenging conditions, such as pentose sugar utilization and the high temperatures required for an efficient simultaneous saccharification and fermentation step. Therefore, a better understanding of the yeast response and adaptation towards the presence of these multiple stresses is of crucial importance to design strategies to improve yeast robustness and bioconversion capacity from lignocellulosic biomass. This review includes an overview of the main inhibitors derived from diverse raw material resultants from different biomass pretreatments, and describes the main mechanisms of yeast response to their presence, as well as to the presence of stresses imposed by xylose utilization and high-temperature conditions, with a special emphasis on the synergistic effect of multiple inhibitors/stressors. Furthermore, successful cases of tolerance improvement of S. cerevisiae are highlighted, in particular those associated with other process-related physiologically relevant conditions. Decoding the overall yeast response mechanisms will pave the way for the integrated development of sustainable yeast cell-based biorefineries.

Sorbic acid stress activates the Candida glabrata high osmolarity glycerol MAP kinase pathway

Weak organic acids such as sorbic acid are important food preservatives and powerful fungistatic agents. These compounds accumulate in the cytosol and disturb the cellular pH and energy homeostasis. Candida glabrata is in many aspects similar to Saccharomyces cerevisiae. However, with regard to confrontation to sorbic acid, two of the principal response pathways behave differently in C. glabrata. In yeast, sorbic acid stress causes activation of many genes via the transcription factors Msn2 and Msn4. The C. glabrata homologs CgMsn2 and CgMsn4 are apparently not activated by sorbic acid. In contrast, in C. glabrata the high osmolarity glycerol (HOG) pathway is activated by sorbic acid. Here we show that the MAP kinase of the HOG pathway, CgHog1, becomes phosphorylated and has a function for weak acid stress resistance. Transcript profiling of weak acid treated C. glabrata cells suggests a broad and very similar response pattern of cells lacking CgHog1 compared to wild type which is over lapping with but distinct from S. cerevisiae. The PDR12 gene was the highest induced gene in both species and it required CgHog1 for full expression. Our results support flexibility of the response cues for general stress signaling pathways, even between closely related yeasts, and functional extension of a specific response pathway.

Resistance of yeasts to weak organic acid food preservatives

Carboxylate weak acids are invaluable for large-scale food and beverage preservation. However, in response to safety concerns, there is now desire to reduce the use of these additives. The resistance to these compounds displayed by spoilage yeasts and fungi is a major reason why these preservatives often have to be used in millimolar levels. This chapter summarizes the mechanisms whereby yeasts are rendered resistant to acetate, propionate, sorbate, and benzoate. In baker's yeast (Saccharomyces cerevisiae), resistance to high acetic acid is acquired partly by loss of the plasma membrane aquaglyceroporin that facilitates the passive diffusional entry of undissociated acid into cells (Fps1), and partly through a transcriptional response mediated by the transcription factor Haa1. Other carboxylate preservatives are too large to enter cells through the Fps1 channel but instead penetrate at appreciable rates by passive diffusion across the plasma membrane. In Saccharomyces and Candida albicans though not, it seems, in the Zygosaccharomyces, resistance to the latter acids involves activation of the War1 transcription factor, which in turn generates strong induction of a specific plasma membrane ATP-binding cassette transporter (Pdr12). The latter actively pumps the preservative anion from the cell. Other contributors to weak acid resistance include enzymes that allow preservative degradation, members of the Tpo family of major facilitator superfamily transporters, and changes to the cell envelope that minimize the diffusional entry of the preservative into the cell.