A petite obligate mutant of Saccharomyces cerevisiae -: Functional mtDNA is lethal in cells lacking the δ subunit of mitochondrial F1-atpase

Within the mitochondrial F1F0-ATP synthase, the nucleus-encoded delta-F1 subunit plays a critical role in coupling the enzyme proton translocating and ATP synthesis activities. In Saccharomyces cerevisiae, deletion of the delta subunit gene ( Delta delta) was shown to result in a massive destabilization of the mitochondrial genome ( mitochondrial DNA; mtDNA) in the form of 100% rho(-)/rho degrees petites ( i.e. cells missing a large portion (> 50%) of the mtDNA ( rho(-)) or totally devoid of mtDNA ( rho degrees)). Previous work has suggested that the absence of complete mtDNA ( rho(+)) in Delta delta yeast is a consequence of an uncoupling of the ATP synthase in the form of a passive proton transport through the enzyme ( i. e. not coupled to ATP synthesis). However, it was unclear why or how this ATP synthase defect destabilized the mtDNA. We investigated this question using a nonrespiratory gene ( ARG8(m)) inserted into the mtDNA. We first show that retention of functional mtDNA is lethal to Delta delta yeast. We further show that combined with a nuclear mutation ( Delta atp4) preventing the ATP synthase proton channel assembly, a lack of delta subunit fails to destabilize the mtDNA, and rho(+) Delta delta cells become viable. We conclude that Delta delta yeast cannot survive when it has the ability to synthesize the ATP synthase proton channel. Accordingly, the rho(-)/rho degrees mutation can be viewed as a rescuing event, because this mutation prevents the synthesis of the two mtDNA-encoded subunits ( Atp6p and Atp9p) forming the core of this channel. This is the first report of what we have called a petite obligate mutant of S. cerevisiae.