Sequence, organization, and evolution of the A+T region of Drosophila melanogaster mitochondrial DNA

The long (4.6-kb) A+T region of Drosophila melanogaster mitochondrial DNAhas been cloned and sequenced. The A+T region is organized in two largearrays of tandemly repeated DNA sequence elements, with nonrepetitiveintervening and flanking sequences comprising only 22% of its length. Thefirst repeat array consists of five repeats of 338-373 bp. The secondconsists of four intact 464-bp repeats and a fifth partial repeat of 137bp. Three DNA sequence elements are found to be highly conserved in D.melanogaster and in several Drosophila species with short A+T regions.These include a 300-bp DNA sequence element that overlaps the DNAreplication origin and two thymidylate stretches identified on opposite DNAstrands. We conclude that the length heterogeneity observed in the A+Tregulatory region in mitochondrial DNAs from the genus Drosophila resultsfrom the expansion (and contraction) of the number of repeated DNA sequenceelements. We also propose that the 300-bp conserved DNA sequence element,in conjunction with another primary sequence determinant, perhaps theadjacent thymidylate stretch, functions in the regulation of mitochondrialDNA replication.