Summary: Partial suppressors of a mitochondrially inherited mutation, [], conferring cold-sensitivity at 20 C were identified. These mapped at one mitochondrial and four unlinked nuclear loci. Most suppressors partially restored the cytochrome deficiency of the cold-sensitive strain at 20 C. Strains carrying two or more suppressors and [] showed considerably impaired growth. This effect was temperature-dependent, being more severe at 37 C, and was not expressed in the presence of the [ ] allele. The cytochrome oxidase activity of one of these strains was no more heat-sensitive than that of the wild-type implying that these mutations did not directly modify cytochrome oxidase. The wild-type strain grown in the presence of chloramphenicol and the cold-sensitive strain grown at 20 C had similar cytochrome spectra and mitochondrial membrane protein profiles on sodium dodecyl sulphate gradient acrylamide gels. [] conferred pleiotropically a low level of resistance to paromomycin at 37 C. It is suggested that [] and the suppressors act at the level of the mitochondrial ribosome.


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