MTO1 CODES FOR A MITOCHONDRIAL PROTEIN REQUIRED FOR RESPIRATION IN PAROMOMYCIN-RESISTANT MUTANTS OF SACCHAROMYCES-CEREVISIAE

Mutations in MTO1 express a respiratory defect only in the context of a mitochondrial genome with a paromomycin-resistance allele, This phen otype is similar to that described previously for mss1 mutants by Deco ster, E. Vassal, A., and Faye, G.(1993) J. Mol. Biol, 232, 79-88, We p resent evidence that Mto1p and Mss1p are mitochondrial proteins and th at they form a heterodimer complex. In a paromomycin-resistant backgro und, mss1 and mto1 mutants are inefficient in processing the mitochond rial COX1 transcript for subunit 1 of cytochrome oxidase, The mutants also fail to synthesize subunit 1 and show a pleiotropic absence of cy tochromes a, a(3), and b. In vivo pulse labeling of an mto1 mutant, ho wever, indicate increased rates of synthesis of other mitochondrial tr anslation products. The respiratory defective phenotype of mto1 and ms s1 mutants is not seen in a paromomycin-sensitive genetic background. The visible absorption spectra of such strains indicate a higher ratio of cytochromes bla and elevated NADH- and succinate-cytochrome c redu ctase activities. To explain these phenotypic characteristics, we prop osed that the Mto1p.Mss1p complex plays a role in optimizing mitochond rial protein synthesis in yeast, possibly by a proofreading mechanism.