The Deletion of the Succinate Dehydrogenase Gene KlSDH1 in Kluyveromyces lactis Does Not Lead to Respiratory Deficiency

We have isolated aKluyveromyces lactis mutant unable to grow on all respiratory carbon sources with the exception of lactate. Functional complementation of this mutant led to the isolation ofKlSDH1 , the gene encoding the flavoprotein subunit of the succinate dehydrogenase (SDH) complex, which is essential for the aerobic utilization of carbon sources. Despite the high sequence conservation of theSDH genes inSaccharomyces cerevisiae andK. lactis , they do not have the same relevance in the metabolism of the two yeasts. In fact, unlikeSDH1 ,KlSDH1 was highly expressed under both fermentative and nonfermentative conditions. In addition to this, but in contrast withS. cerevisiae ,K. lactis strains lackingKlSDH1 were still able to grow in the presence of lactate. In these mutants, oxygen consumption was one-eighth that of the wild type in the presence of lactate and was normal with glucose and ethanol, indicating that the respiratory chain was fully functional. Northern analysis suggested that alternative pathway(s), which involves pyruvate decarboxylase and the glyoxylate cycle, could overcome the absence of SDH and allow (i) lactate utilization and (ii) the accumulation of succinate instead of ethanol during growth on glucose.