A Klaac null mutant of Kluyveromyces lactis is complemented by a single copy of the Saccharomyces cerevisiae AAC1 gene.

The KlAAC gene, encoding the ADP/ATP carrier in Kluveromyces lactis, has previously been cloned by complementation of the op1(aac2) mutation of Saccharomyces cerevisiae. We examined the effect of a null mutation of this gene on the phenotype of K. lactis. The consequence of this mutation was found to be multiple. The mutant was respiratory deficient, had an undetectable level of cytochrome a-a3 and b and did not grow on glycerol. The mitochondrial D-lactate ferricytochrome c oxidoreductase activity, as well as the lactate-induced transcription of its gene, KlDLD, was severely reduced. Furthermore, the mutant was unable to grow on galactose, maltose and raffinose. Transcript analysis showed that KlAAC was the only ADP/ATP carrier gene present in K. lactis. The Klaac mutation was fully complemented not only by AAC2, the major gene for the ADP/ATP carrier in S. cerevisiae, but also by AAC1, a gene which is poorly expressed in S. cerevisiae. AAC1 introduced in K. lactis was transcribed to a high level consistent with normal growth on glycerol being restored in the transformed mutant. KlAAC was not subject to control by KlHap2, in contrast to AAC2 which is regulated by the Hap2 complex in S. cerevisiae.