Order and orientation of genic sequences in circular mitochondrial DNA from Saccharomyces exiguus: implications for evolution of yeast mtDNAs.

Mapping of sequences specifying the large and small ribosomal RNAs and six polypeptides in the circular 23.7 kbp mitochondrial DNA of Saccharomyces exiguus has shown that these genes have the same orientation and that a 5 gene cluster is common to this DNA and the 18.9 kbp mtDNA from Torulopsis glabrata. Included in the preserved region are juxtaposed sequences specifying ATPase subunits 6 and 9 which have the same order and orientation as analogous genes in the Escherichia coli unc operon. The above data, together with knowledge that these two sequences are dispersed in larger yeast mtDNAs, leads us to suggest that larger forms are derived from a smaller ancestral molecule that would have had some resemblance to the mtDNAs of S. exiguus and T. glabrata.