Glycerol is one of the main byproducts in ethanol fermentation due to its importance in redox balance and response to osmotic stress in Saccharomyces cerevisiae. Since its production diverts carbon from alcohol production, traditional gene-editing methods have been applied to the glycerol synthesis pathway. However, such approaches generate undesirable phenotypes for industrial applications. In the present study, we employed the CRISPR-dCas9 system to moderately downregulate the expression of GPD1 and GPD2, the two main genes involved in this metabolism. GPD2 gene expression downregulation and a graded reduction in glycerol production after repression of four different target sites in each paralogue were achieved. Employment of the CRISPRi approach for GPD gene modulation resulted in higher specific ethanol productivity (SEP) than that of single knockout cells. Targeted modulation in a region −140 basepairs upstream of the transcription start site (TSS) of GPD1 resulted in a 3% increase in ethanol production compared to the wild type and gpd Δ strains. Such regulation, combined with GPD2 deletion, revealed the higher SEP among all tested strains. Furthermore, a GPD1-modulated strain maintained tolerance to high osmolarity in very high-gravity (VHG) fermentation while maintaining its ethanol production levels above those observed in the control strain.
Spavieri et al. (Thu,) studied this question.