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 th e stability of wild-type (p(+)) mitochondrial DNA (mtDNA) and plays an important role in mtDNA organization. Here ive report new functions f or Abf2p in mtDNA transactions. We find that in homozygous Delta abf2 crosses, the pattern of sorting of mtDNA and mitochondrial matrix prot ein is altered, and mtDNA recombination is suppressed relative to homo zygous ABF2 crosses. Although Abf2p is known to be required for the ma intenance of mtDNA in p(+) cells growing on rich dextrose medium, ive find that it is not required for the maintenance of mtDNA in p(-) cell s grown on the same medium. The content of both p(+) and p(-) mtDNAs i s increased in cells by 50-150% by moderate (two- to threefold) increa ses in the ABF2 copy number, suggesting that Abf2p plays a role in mtD NA copy control. Overproduction of Abf2p by greater than or equal to 1 0-fold from an ABF2 placed under the control of the GAL1 promoter, how ever, leads to a rapid loss of p(+) mtDNA and a quantitative conversio n of pi cells to petites within two to four generations after a shift of the culture from glucose to galactose medium. Overexpression of Abf 2p in p-cells also leads to a loss of mtDNA, but at a slower rate than was observed for pi cells. The mtDNA instability phenotype is related to the DNA-binding properties of Abf2p because a mutant Abf2p that co ntains mutations in residues of both HMG box domains known to affect D NA binding in vitro, and that binds poorly to mtDNA in vivo, complemen ts Delta abf2 cells only weakly and greatly lessens the effect of over production on mtDNA instability. In vivo binding was assessed by coloc alization to mtDNA of fusions between mutant or wild-type Abf2p and gr een fluorescent protein. These findings are discussed in the contest o f a model relating mtDNA cop! number control and stability to mtDNA re combination.