SEQUENCE, ORGANIZATION, AND EVOLUTION OF THE A-MELANOGASTER MITOCHONDRIAL-DNA(T REGION OF DROSOPHILA)

The long (4.6-kb) A+T region of Drosophila melanogaster mitochondrial DNA has been cloned and sequenced. The A+T region is organized in two large arrays of tandemly repeated DNA sequence elements, with nonrepet itive intervening and flanking sequences comprising only 22% of its le ngth. The first repeat array consists of five repeats of 338-373 bp. T he second consists of four intact 464-bp repeats and a fifth partial r epeat of 137 bp. Three DNA sequence elements are found to be highly co nserved in D. melanogaster and in several Drosophila species with shor t A+T regions. These include a 300-bp DNA sequence element that overla ps the DNA replication origin and two thymidylate stretches identified on opposite DNA strands. We conclude that the length heterogeneity ob served in the A+T regulatory region in mitochondrial DNAs from the gen us Drosophila results from the expansion (and contraction) of the numb er of repeated DNA sequence elements. We also propose that the 300-bp conserved DNA sequence element, in conjunction with another primary se quence determinant, perhaps the adjacent thymidylate stretch, function s in the regulation of mitochondrial DNA replication.