Translational readthrough of a termination codon in the yeast mitochondrial mRNA VAR1 as a result of mutation in the release factor mRF1.

Yeast mitochondrial DNA codes for eight major polypeptides. Translation of he mitochondrially encoded polypeptides in strains with mutated mitochondrial release factor, mRF1, was found to result in the synthesis of a novel protein, V2. Different mrf1 alleles were associated with different efficiency of V2p synthesis. Translation of V2p was enhanced by paromomycin. Comparative analysis of peptides resulting from protease digestion indicated that V2p is a derivative of Var1p. According to our hypothesis, V2p represents a readthrough product of the natural stop codon in VAR1 mRNA.

The mitochondrial genome of Debaryomyces (Schwanniomyces) occidentalis encodes subunits of NADH dehydrogenase complex I.

nad genes encoding subunits of the NADH dehydrogenase complex 1 have been revealed in the yeast Debaryomyces (Schwanniomyces) occidentalis. nad1, nad3, nad5, nad6 and most large mitochondrial genes have been located on a circular 41-kb map of mitochondrial DNA from this petite negative species. The genes nad1-nad6 are co-transcribed and the transcription is not inhibited by glucose. Sequences of nad6 and 5'-nad1 compared to homologs in other yeasts indicate better amino acids conservation for nad1 product than for nad6. A cytochrome b deficient mutant dependent on alternative oxidase and functional complex 1 for growth on respirable substrates also exhibits co-transcription of nad1-nad6.

The mitochondrial genome can be altered or lost without lethal effect in the petite-negative yeast Debaryomyces (Schwanniomyces) occidentalis.

The nature of mutations affecting several cytochrome-deficient mutants of Debaryomyces (Schwanniomyces) occidentalis has been characterized. The DR12 mutant, which is deficient in cytochrome b, and the B10Mn mutant, which is deficient in cytochromes b and a, a3, are deleted in the mitochondrial CYB and COX1 genes respectively. The B10 strain, which is partially deficient in cytochrome b, has no detectable change in its mitochondrial DNA and possibly carries nuclear lesion(s). These three mutants, unlike the rho(-) and rho degrees "petite" mutants of Saccharomyces cerevisiae, can still grow on non-fermentable substrates, due to the development of a salicylhydroxamic acid (SHAM)-sensitive alternative pathway linked to phosphorylation at site 1. A gly(-) mutant lacking mtDNA and respiratory capacity has been isolated. For the first time, it is demonstrated that mtDNA can be altered or even lost without lethal consequence in D. occidentalis, although this yeast was classified as a petite-negative species.