Mutations in MTO1 express a respiratory defect only in the context of a mitochondrial genome with a paromomycin-resistance allele. This phenotype is similar to that described previously for mss1 mutants by Decoster, E., Vassal, A., and Faye, G. (1993) J. Mol. Biol. 232, 79-88. We present evidence that Mto1p and Mss1p are mitochondrial proteins and that they form a heterodimer complex. In a paromomycin-resistant background, mss1 and mto1 mutants are inefficient in processing the mitochondrial COX1 transcript for subunit 1 of cytochrome oxidase. The mutants also fail to synthesize subunit 1 and show a pleiotropic absence of cytochromes a, a3, and b. In vivo pulse labeling of an mto1 mutant, however, indicate increased rates of synthesis of other mitochondrial translation products. The respiratory defective phenotype of mto1 and mss1 mutants is not seen in a paromomycin-sensitive genetic background. The visible absorption spectra of such strains indicate a higher ratio of cytochromes b/a and elevated NADH- and succinate-cytochrome c reductase activities. To explain these phenotypic characteristics, we proposed that the Mto1p · Mss1p complex plays a role in optimizing mitochondrial protein synthesis in yeast, possibly by a proofreading mechanism.
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