With the aim to reduce fermentation by-products and to promote respiratory metabolism by shifting the fermentative/oxidative balance we evaluated the constitutive overexpression of the genes SAK1 and HAP4 in Saccharomyces cerevisiae. Sak1p is one of three kinases responsible for phosphorylation, and thereby activation, of the Snf1p complex, while Hap4p is the activator subunit of the transcriptional complex Hap2/3/4/5. We compared the physiology of a SAK1 overexpressing strain with a strain overexpressing the gene HAP4 in wild-type and sdh2 deletion (respiratory deficient) background. Both, SAK1 and HAP4 overexpression led to upregulation of glucose-repressed genes and to reduced by-product formation rates (ethanol and glycerol). SAK1 overexpression had a greater impact on growth rates than HAP4 overexpression. Elevated transcript levels of SAK1, but not HAP4, resulted in increased biomass yield in batch cultures grown on glucose (aerobic, excess glucose) as well as on non-fermentable carbon sources. SAK1 overexpression, but not the combined overexpression of SAK1 and HAP4 or overexpression of HAP4 alone, restored growth on ethanol in a sdh2 deletion strain. In glucose-grown shake flask cultures, the sdh2 deletion strain with SAK1 and HAP4 overexpression produced succinic acid at a titer of 8.5 g L(-1) and a yield of 0.26 mol (mol glucose)(-1) within 216 hours. We here report for the first time that a constitutive high expression of SAK1 alleviates glucose repression and shifts the fermentative/oxidative balance under both glucose- repressed and derepressed conditions.
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