TRANSCRIPTION-DEPENDENT DNA TRANSACTIONS IN THE MITOCHONDRIAL GENOME OF A YEAST HYPERSUPPRESSIVE PETITE MUTANT

Mitochondrial DNA (mtDNA) of Saccharomyces cerevisiae contains highly conserved sequences, called rep/ori, that are associated with several aspects of its metabolism. These rep/ori sequences confer the transmis sion advantage exhibited by a class of deletion mutants called hypersu ppressive petite mutants. In addition, because they share features wit h the mitochondrial leading-strand DNA replication origin of mammals, rep/ori sequences have also been proposed to participate in mtDNA repl ication initiation. Like the mammalian origins, where transcription is used as a priming mechanism for DNA synthesis, yeast rep/ori sequence s contain an active promoter. Although transcription is required for m aintenance of wild-type mtDNA in yeast, the role of the rep/ori promot er as a cis-acting element involved in the replication of wild-type mt DNA is unclear, since mitochondrial deletion mutants need neither tran scription nor a rep/ori sequence to maintain their genome. Similarly, transcription from the rep/ori promoter does not seem to be necessary for biased inheritance of mtDNA, As a step to elucidate the function o f the rep/ori promoter, we have attempted to detect transcription-depe ndent DNA transactions in the mtDNA of a hypersuppressive petite mutan t. We have examined the mtDNA of the well-characterized petite mutant a-1/1R/Z1, whose repeat unit shelters the rep/ori sequence ori1, in st rains carrying either wild-type or null alleles of the nuclear genes e ncoding the mitochondrial transcription apparatus. Complex DNA transac tions were detected that take place around GC-cluster C, an evolutiona rily conserved GC-rich sequence block immediately downstream from the rep/ori promoter. These transactions are strictly dependent upon mitoc hondrial transcription.