Organization of Achlya mtDNA: a population with two orientations and a large inverted repeat containing the rRNA genes.

Abstract

MtDNA [mitochondrial DNA] organization in the oomycetous water mold Achlya was investigated. This primitive organism contains a circular mitochondrial genome of 49.8 kilobase [kb] pairs. Extensive restriction endonuclease analysis of indicates that a significant portion of the genome is present as an inverted repeat. Of 52 restriction sites for 14 enzymes thus far mapped, 28 sites cluster in two 9.6-kb pair regions; within these regions, the sequence of sites is inverted but the spacing between analogous sites is identical. The repeat arms have a maximum length of 12.1 kb pairs and are separated by 4.6-8.4 and 21.0-22.3 kb pairs of unique sequences. Transfer hybridization experiments show that genes for both the large and the small rRNA are contained within each repeat. Restriction endonuclease analysis shows that the unique regions between the inverted repeats are present in both possible orientations with respect to each other and in approximately equal proportions. These orientational, or flip-flop, isomers of the unique regions are postulated to occur by intramolecular homologous recombination between the repeated regions.