Human mitochondrial DNA: analysis of 7S DNA from the origin of replication.

Abstract

Heat-treated samples of human mitochondrial [Hela human cervical cancer cells] DNA (mtDNA) exhibited a set of 3 low MW DNA bands in addition to the major mtDNA band when electrophoresed in polyacrylamide gels. These DNA components were seen only after heat treatment or after relaxation of the mtDNA with a restriction endonuclease. The 3 components were single stranded and had sizes of 550, 585 and 620 nucleotides, close to the size (600 nucleotides) estimated from contour length measurements for the 7S DNA from the D loop of human mtDNA. Hybridization of the components with restriction endonuclease fragments of known position in the mtDNA confirmed this identification. Digestion of each 7S DNA component with the restriction endonuclease Hae III produced 3 fragments, 2 of which were identical in size among the components and the 3rd of which varied. This 3rd fragment, from the 5'' end of each component, differed in length by .apprx. 35 nucleotides among the components. Human 7S mtDNA synthesis is probably terminated at a distinct position, and it is either initiated at 1 of 3 possible sites in the same mtDNA or the mtDNA population consists of 3 subpopulations, each differing from the others by the presence or absence of a nucleotide sequence immediately adjacent to the to the origin of replication.