Aims: To identify yeast mutants that show a strong redox dependence of the ability to respire, we systematically screened a yeast deletion library for mutants that require the presence of reductants for growth on nonfermentable carbon sources.
Results: Respirative growth of 44 yeast mutants was significantly improved by the addition of dithiothreitol or glutathione. Two mutants that were strongly stimulated by reductants lacked the proteins Cmc1 and Coa4. Both proteins belong to the family of "twin Cx(9)C" proteins present in the intermembrane space of mitochondria. Deletion of CMC1 or COA4 leads to assembly defects of cytochrome c oxidase, in particular to the lack of Cox1 and rapid degradation of Cox2 and Cox3. Interestingly, the presence of the reductants does not suppress these assembly defects and the levels of cytochrome c oxidase remain reduced. Reductants and antioxidants such as ascorbic acid rather counteract the effects of hydrogen peroxide that is produced from partially assembled cytochrome c oxidase intermediates.
Innovation: Here we show that oxidative stress generated by the accumulation of partially assembled respiratory chain complexes prevents growth on carbon sources that force cells to respire.
Conclusion: Defects in the assembly of cytochrome c oxidase can lead to increased production of hydrogen peroxide, which is sensed in cells and blocks their proliferation. We propose that this redox-regulated feedback regulation specifically slows down the propagation of cells carrying respiratory chain mutations in order to select for cells of high mitochondrial fitness.