Functions of the High Mobility Group Protein, Abf2p, in Mitochondrial DNA Segregation, Recombination and Copy Number in Saccharomyces cerevisiae

Previous studies have established that the mitochondrial high mobility group (HMG) protein, Abf2p, of Saccharomyces cerevisiae influences the stability of wild-type (ρ+) mitochondrial DNA (mtDNA) and plays an important role in mtDNA organization. Here we report new functions for Abf2p in mtDNA transactions. We find that in homozygous Δabf2 crosses, the pattern of sorting of mtDNA and mitochondrial matrix protein is altered, and mtDNA recombination is suppressed relative to homozygous ABF2 crosses. Although Abf 2p is known to be required for the maintenance of mtDNA in ρ+ cells growing on rich dextrose medium, we find that it is not required for the maintenance of mtDNA in ρ− cells grown on the same medium. The content of both ρ+ and ρ− mtDNAs is increased in cells by 50–150% by moderate (two- to threefold) increases in the ABF2 copy number, suggesting that Abf2p plays a role in mtDNA copy control. Overproduction of Abf 2p by ≥10-fold from an ABF2 gene placed under control of the GAL1 promoter, however, leads to a rapid loss of ρ+ mtDNA and a quantitative conversion of ρ+ cells to petites within two to four generations after a shift of the culture from glucose to galactose medium. Overexpression of Abf2p in ρ− cells also leads to a loss of mtDNA, but at a slower rate than was observed for ρ+ cells. The mtDNA instability phenotype is related to the DNA-binding properties of Abf 2p because a mutant Abf 2p that contains mutations in residues of both HMG box domains known to affect DNA binding in vitro, and that binds poorly to mtDNA in vivo, complements Δabf2 cells only weakly and greatly lessens the effect of overproduction on mtDNA instability. In vivo binding was assessed by colocalization to mtDNA of fusions between mutant or wild-type Abf 2p and green fluorescent protein. These findings are discussed in the context of a model relating mtDNA copy number control and stability to mtDNA recombination.