Transcription-Dependent DNA Transactions in the Mitochondrial Genome of a Yeast Hypersuppressive Petite Mutant

Mitochondrial DNA (mtDNA) ofSaccharomyces cerevisiae contains highly conserved sequences, calledrep/ori , that are associated with several aspects of its metabolism. Theserep/ori sequences confer the transmission advantage exhibited by a class of deletion mutants called hypersuppressive petite mutants. In addition, because they share features with the mitochondrial leading-strand DNA replication origin of mammals,rep/ori sequences have also been proposed to participate in mtDNA replication initiation. Like the mammalian origins, where transcription is used as a priming mechanism for DNA synthesis, yeastrep/ori sequences contain an active promoter. Although transcription is required for maintenance of wild-type mtDNA in yeast, the role of therep/ori promoter as acis -acting element involved in the replication of wild-type mtDNA is unclear, since mitochondrial deletion mutants need neither transcription nor arep/ori sequence to maintain their genome. Similarly, transcription from therep/ori promoter does not seem to be necessary for biased inheritance of mtDNA. As a step to elucidate the function of therep/ori promoter, we have attempted to detect transcription-dependent DNA transactions in the mtDNA of a hypersuppressive petite mutant. We have examined the mtDNA of the well-characterized petite mutant a-1/1R/Z1, whose repeat unit shelters therep/ori sequenceori1 , in strains carrying either wild-type or null alleles of the nuclear genes encoding the mitochondrial transcription apparatus. Complex DNA transactions were detected that take place around GC-cluster C, an evolutionarily conserved GC-rich sequence block immediately downstream from therep/ori promoter. These transactions are strictly dependent upon mitochondrial transcription.