Summary: The gene, encoding the E1α subunit of the mitochondrial pyruvate-dehydrogenase (PDH) complex was isolated from a genomic library by screening with a 1·1 kb internal fragment of the gene. The predicted amino acid sequence encoded by showed 87% similarity and 79% identity to its counterpart. Disruption of resulted in complete absence of PDH activity in cell extracts. The maximum specific growth rate on glucose of null mutants was 3·5-fold lower than that of the wild-type, whereas growth on ethanol was unaffected. Wild-type CBS 2359 exhibits a Crabtree-negative phenotype, i.e. no ethanol was produced in aerobic batch cultures grown on glucose. In contrast, substantial amounts of ethanol and acetaldehyde were produced in aerobic cultures of an isogenic null mutant. A wild-type specific growth rate was restored after introduction of an intact gene but not, as previously found for mutants, by cultivation in the presence of leucine. The occurrence of aerobic fermentation and slow growth of the null mutant indicate that, although present, the enzymes of the PDH bypass (pyruvate decarboxylase, acetaldehyde dehydrogenase and acetyl-CoA synthetase) could not efficiently replace the PDH complex during batch cultivation on glucose. Only at relatively low growth rates ( = 0·10 h) in aerobic, glucose-limited chemostat cultures, could the PDH bypass completely replace the PDH complex, thus allowing fully respiratory growth. This resulted in a lower biomass yield [g biomass (g glucose)] than in the wild-type due to a higher consumption of ATP in the PDH bypass compared to the formation of acetyl-CoA via the PDH complex.


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