A single mutation in the first transmembrane domain of yeast COX2 enables its allotopic expression

During the course of evolution, a massive reduction of the mitochondrial genome content occurred that was associated with transfer of a large number of genes to the nucleus. To further characterize factors that control the mitochondrial gene transfer/retention process, we have investigated the barriers to transfer of yeastCOX2 , a mitochondrial gene coding for a subunit of cytochromec oxidase complex. Nuclear-recodedSaccharomyces cerevisiae COX2 fused at the amino terminus to various alternative mitochondrial targeting sequences (MTS) fails to complement the growth defect of a yeast strain with an inactivated mitochondrialCOX2 gene, even though it is expressed in cells. Through random mutagenesis of one such hybridMTS-COX2 , we identified a single mutation in the first Cox2 transmembrane domain (W56 → R) that (i ) results in the cellular expression of a Cox2 variant with a molecular mass indicative of MTS cleavage, which (ii ) supports growth of acox2 mutant on a nonfermentable carbon source, and that (iii ) partially restores cytochromec oxidase-specific respiration by the mutant mitochondria.COX2W56R can be allotopically expressed with an MTS derived fromS. cerevisiae OXA1 orNeurospora crassa SU9 , both coding for hydrophobic mitochondrial proteins, but not with an MTS derived from the hydrophilic protein Cox4. In contrast to some other previously transferred genes, allotopicCOX2 expression is not enabled or enhanced by a 3′-UTR that localizes mRNA translation to the mitochondria, such as yeastATP23 ′-UTR . Application of in vitro evolution strategies to other mitochondrial genes might ultimately lead to yeast entirely lacking the mitochondrial genome, but still possessing functional respiratory capacity.