An enzyme in yeast mitochondria that catalyzes a step in branched‐chain amino acid biosynthesis also functions in mitochondrial DNA stability.

The yeast mitochondrial high mobility group protein Abf2p is required, under certain growth conditions, for the maintenance of wild‐type (rho+) mitochondrial DNA (mtDNA). We have identified a multicopy suppressor of the mtDNA instability phenotype of cells with a null allele of the ABF2 gene (delta abf2). The suppressor is a known gene, ILV5, encoding the mitochondrial protein, acetohydroxy acid reductoisomerase, which catalyzes a step in branched‐chain amino acid biosynthesis. Efficient suppression occurs with just a 2‐ to 3‐fold increase in ILV5 copy number. Moreover, in delta abf2 cells with a single copy of ILV5, changes in mtDNA stability correlate directly with changes in conditions that are known to affect ILV5 expression. Wild‐type mtDNA is unstable in cells with an ILV5 null mutation (delta ilv5), leading to the production of mostly rho‐ petite mutants. The instability of rho+ mtDNA in delta ilv5 cells is not simply a consequence of a block in branched‐chain amino acid biosynthesis, since mtDNA is stable in cells with a null allele of the ILV2 gene, which encodes another enzyme of that pathway. The most severe instability of rho+ mtDNA is observed in cells with null alleles of both ABF2 and ILV5. We suggest that ILV5 encodes a bifunctional protein required for branched‐chain amino acid biosynthesis and for the maintenance of rho+ mtDNA.