Respiration deficient mutants in the A+T-rich region on yeast mitochondrial DNA containing the var1 gene

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Abstract
Several mit mutants mapping within or near the var1 determinant region have been characterized genetically and biochemically. These mutants were isolated using a new enrichment protocol which simplifies the isolation and identification of rare respiration-deficient mutants of yeast. Two of the mutants, PZ200L and PZ206, map in genome segments which flank the known varl gene reading frame; nevertheless, both belong to the same complementation group, apparently that of the varl gene. A third mutant, PZ200R is closely linked to one of the varl allelic determinants now known to be a short insertion within the gene. All three var1 mutants exhibit decreased levels of mitochondrial protein synthesis and negligible activity of the respiratory enzyme complexes. Another cluster of mutants belonging to a separate complementation group from that defined by PZ200L and PZ206 was also mapped and it contains mutants in the nearby serine tRNA gene. The isolation of these mutants in the varl region shows that the varl locus contains information essential for the maintenance of respiration-competent mitochondria. Because these mutants affect mitochondrial protein synthesis, their existence supports the previous hypothesis that the varl protein is an integral component of mitochondrial ribosomes. Furthermore, the mutant sites are present in a DNA sequence that is highly, rich in A+T residues that also contains a gene. Since approximately 50% of the yeast mitochondrial genome is similarly rich in A+T and since most of those regions have not yet been sequenced it is quite possible that other A+T-rich genes may exist.