Evolutionary and reverse engineering to increase Saccharomyces cerevisiae tolerance to acetic acid, acidic pH, and high temperature

Saccharomyces cerevisiae scarcely grows on minimal media with acetic acid, acidic pH, and high temperatures. In this study, the adaptive laboratory evolution (ALE), whole-genome analysis, and reverse engineering approaches were used to generate strains tolerant to these conditions. The thermotolerant strain TTY23 and its parental S288C were evolved through 1 year, in increasing concentrations of acetic acid up to 12 g/L, keeping the pH <= 4. Of the 18 isolated strains, 9 from each ancestor, we selected the thermo-acid tolerant TAT12, derived from TTY23, and the acid tolerant AT22, derived from S288C. Both grew in minimal media with 12 g/L of acetic acid, pH 4, and 30 degrees C, and produced ethanol up to 29.25 +/- 6 mmol/g(DCW)/h-neither of the ancestors thrived in these conditions. Furthermore, only the TAT12 grew on 2 g/L of acetic acid, pH 3, and 37 degrees C, and accumulated 16.5 +/- 0.5 mmol/g(DCW)/h of ethanol. Whole-genome sequencing and transcriptomic analysis of this strain showed changes in the genetic sequence and transcription of key genes involved in the RAS-cAMP-PKA signaling pathway (RAS2, GPA2, and IRA2), the heat shock transcription factor (HSF1), and the positive regulator of replication initiation (SUM1), among others. By reverse engineering, the relevance of the combined mutations in the genes RAS2, HSF1, and SUM1 to the tolerance for acetic acid, low pH, and high temperature was confirmed. Alone, the RAS2 mutation yielded acid tolerance and HSF1 nutation thermotolerance. Increasing the thermo-acidic niche and acetic acid tolerance of S. cerevisiae can contribute to improve economic ethanol production.

Automated summary

This study used adaptive laboratory evolution (ALE) and whole-genome analysis to generate strains of Saccharomyces cerevisiae tolerant to high temperatures, acetic acid, and acidic pH. Through a year of evolution, two strains were selected, TAT12 and AT22, which were capable of growing and producing ethanol in conditions previously inhibitory to their ancestors. Whole-genome sequencing and transcriptomic analysis showed mutations in key genes related to stress response and replication, confirming the importance of these mutations in enhancing the tolerance of S. cerevisiae to harsh environments.