No abstract

NDI1 is the unique gene encoding the internal mitochondrial NADH dehydrogenase of Saccharomyces cerevisiae. The enzyme catalyzes the transfer of electrons from intramitochondrial NADH to ubiquinone. Surprisingly, NDI1 is not essential for respiratory growth. Here we demonstrate that this is due to in vivo activity of an ethanol-acetaldehyde redox shuttle, which transfers the redox equivalents from the mitochondria to the cytosol. Cytosolic NADH can be oxidized by the external NADH dehydrogenases. Deletion of ADH3, encoding mitochondrial alcohol dehydrogenase, did not affect respiratory growth in aerobic, glucose-limited chemostat cultures. Also, an ndi1⌬ mutant was capable of respiratory growth under these conditions. However, when both ADH3 and NDI1 were deleted, metabolism became respirofermentative, indicating that the ethanolacetaldehyde shuttle is essential for respiratory growth of the ndi1⌬ mutant. In anaerobic batch cultures, the maximum specific growth rate of the adh3⌬ mutant (0.22 h ؊1 ) was substantially reduced compared to that of the wild-type strain (0.33 h ؊1 ). This is consistent with the hypothesis that the ethanol-acetaldehyde shuttle is also involved in maintenance of the mitochondrial redox balance under anaerobic conditions. Finally, it is shown that another mitochondrial alcohol dehydrogenase is active in the adh3⌬ ndi1⌬ mutant, contributing to residual redox-shuttle activity in this strain.

Analysis of S. cerevisiae cultures with generation times varying between 2 and 35 hours shows that the expression of half of all yeast genes is affected by the specific growth rate.