Adaptive responses of evolved Saccharomyces cerevisiae strains tolerant
to acidic pH, acetic acid, and supraoptimal temperature
Abstract
Ethanol fermentations can be prematurely halted as Saccharomyces
cerevisiae faces adverse conditions, such as acidic pH, presence of
acetic acid, and supraoptimal temperatures. The knowledge on yeast
responses to these conditions is essential to endowing a tolerant
phenotype to another strain by targeted genetic manipulation. In this
study, physiological, genomic, and transcriptomic analyses were
conducted to obtain insights on molecular responses which could
potentially render yeast tolerant towards thermoacidic conditions. To
this end, we used thermotolerant TTY23, acid tolerant AT22, and
thermo-acid tolerant TAT12 strains previously generated. The results
showed an increase in thermoacidic profiles in the tolerant strains. The
whole-genome sequence revealed the importance of genes related to: H
+, iron, and glycerol transport (i.e. FRE1/2,
JEN1, VMA2, VCX1, KHA1, AQY3, and
ATO2); transcriptional regulation of stress responses to drugs,
ROS and heat-shock (i.e. HSF1, SKN7, BAS1,
HFI1, and WAR1); and adjustments of fermentative growth
and stress responses by glucose signaling pathways (i.e., ACS1,
GPA1/2, RAS2, IRA2 and REG1). At 30°C and pH
5.5, more than a thousand differentially expressed genes (DEGs) were
identified in each strain. The integration of results from genomics and
transcriptomics revealed that, as part of their adaptive responses,
evolved yeast strains aimed to adjust their intracellular pH by H
+ and acetic acid transport; modify their metabolism
and stress responses via glucose signaling pathways; control of cellular
ATP pools by regulating translation and de novo synthesis of
nucleotides; and direct the synthesis, folding and rescue of proteins
throughout the heat-shock stress response. Furthermore, tolerant strains
did not show growth trade off under optimal ancestral conditions.
Moreover, the motifs analysis in mutated TFs suggested a significant
association of SFP1, YRR1, BAS1, HFI1, HSF1, and SKN7 TFs with DEGs
found in thermoacidic tolerant yeast strains. This study provides
insights into the molecular elements associated with yeast tolerance to
acidic pH, acetic acid, and supraoptimal temperatures; information that
can be used in inverse metabolic engineering.