Mutations suppressing the effects of a deletion of the Phosphoglucose isomerase gene PGI1 in Saccharomyces cerevisiae

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Abstract
A mutant with a deletion covering the phosphoglucose isomerase gene PGI1, allele pgi1Δ, can only grow on a medium containing fructose and low concentrations of glucose whereas growth is completely inhibited by glucose concentrations higher than 0.4%. This was used to select suppressor mutants restoring growth on synthetic media with 2% glucose as the sole carbon source. One complementation group, SPG1, was defined by recessive mutations. The ability to grow on glucose media was strictly dependent on functional mitochondria. The generation time of the selected mutants on YEP glucose was 6–8 h. No ethanol was formed from glucose and the levels of respiration were very high. These phenotypes were also observed in single pgi1Δ mutants when growing on fructose media supplemented with 0.4% glucose. The other glycolytic enzymes, the enzymes of the glucose-6-phosphate oxidation pathway as well as catabolite repression were normal in suppressed pgi1Δ mutants. The suppressor mutation alone caused no abnormal phenotype. The results suggest that the spg1 suppressor mutations allow S. cerevisiae pgi1Δ mutant strains to grow on glucose by using the Pentose-P cycle in combination with unusual strong respiration.