Homology and structural relationships between the dimeric and monomeric circular forms of mitochondrial DNA from human leukemic leukocytes.

Abstract

Abstract Mitochondrial DNA occurs in multiple circular forms in human leukemic leukocytes. As part of an inquiry into the structural relationships between the forms, the 5- and 10-micron circular duplexes were separated and characterized by sedimentation procedures. The buoyant densities in neutral cesium chloride were indistinguishable, indicating that over-all base compositions of the two species are the same. The buoyant densities of the corresponding heavy and light strands in the two species were also indistinguishable. Together, the results suggest that the dimer consists of two monomer genomes. Reannealing of equal amounts of fragmented light monomer and heavy dimer complementary strands isolated from alkaline buoyant cesium chloride gradients leads to a high molecular weight duplex with a buoyant density indistinguishable from the parent duplexes. It is concluded from this result that the degree of homology between monomer and dimer is at least 90%. Denaturation of a mixture of lightly nicked monomer and dimer duplexes, followed by reannealing in formamide solutions, leads to mixtures of circular monomer and dimer duplexes and heteroduplexes which contain one dimer strand and two monomer strands. These heteroduplexes occur in the form of a “figure 8” molecule, a new DNA structure which we have called the fused dimer. Neither heterologous regions, nor insertions, nor deletions were found in a careful examination of the electron micrographs. It is concluded from this part of the work that heterologous regions, insertions, or deletions exceeding 50 to 100 nucleotides in length do not occur in the heteroduplexes. At this level of insight, the dimer is a circular concatamer of two monomer genomes. In addition, the results show that leukemic leukocyte mitochondrial DNA monomers (as well as dimers) are substantially homogeneous in base sequence. The electron microscope studies also show that monomer genomes in the dimer are connected in a head-to-tail structure, rather than in a head-to-head structure.