A TEST OF THE TRANSCRIPTION MODEL FOR BIASED INHERITANCE OF YEAST MITOCHONDRIAL-DNA

Two strand-specific origins of replication appear to be required for m ammalian mitochondrial DNA (mtDNA) replication. Structural equivalents of these origins are found in the rep sequences of Saccharomyces cere visiae mtDNA, These striking similarities have contributed to a univer sal model for the initiation of mtDNA replication in which a primer is created by cleavage of an origin region transcript, Consistent with t his model are the properties of deletion mutants of yeast mtDNA ([rho( -)]) with a high density of reps (HS [rho(-)]). These mutant mtDNAs ar e preferentially inherited by the progeny resulting from the mating of HS [rho(-)] cells with cells containing wild-type mtDNA ([rho(+)]). T his bias is presumed to result from a replication advantage conferred on HS [rho(-)] mtDNA by the high density of rep sequences acting as or igins. To test whether transcription is indeed required for the prefer ential inheritance of HS [rho(-)] mtDNA, we deleted the nuclear gene ( RPO41) for the mitochondrial RNA polymerase, reducing transcripts by a t least 1000-fold. Since [rho(-)] genomes, but not [rho(+)] genomes, a re stable when RPO41 is deleted, we examined matings between HS [rho(- )] and neutral [rho(-)] cells, Neutral [rho(-)] mtDNAs lack rep sequen ces and are not preferentially inherited in [rho(-)] x [rho(+)] crosse s. In HS [rho(-)] x neutral [rho(-)] matings, the HS [rho(-)] mtDNA wa s preferentially inherited whether both parents were wild type or both were deleted for RPO41. Thus, transcription from the rep promoter doe s not appear to be necessary for biased inheritance, Our results, and analysis of the literature, suggest that priming by transcription is n ot a universal mechanism for mtDNA replication initiation.