ABSTRACT
Mammalian mitochondrial DNA often contains a short DNA displacement loop at the heavy-strand origin of replication. This short nascent DNA molecule has been used to study site-specific termination of mitochondrial DNA synthesis in human and mouse cells. We examined D-loop strand termination in two distantly related artiodactyls, the pig and the cow. Porcine mitochondrial DNA was unique among mammals in that it contained only a single species of D-loop single-stranded DNA. Its 3' end mapped to a site 187 nucleotides from the 5' end of the proline tRNA gene. This site was 21 and 47 nucleotides 5' to two very similar sequences (5' ACATATPyATTAT 3') which are closely related to the human and mouse termination-associated sequences noted by Doda et al. (J. N. Doda, D. T. Wright, and D. A. Clayton, Proc. Nat. Acad. Sci. USA 78:616-6120, 1981). Bovine mitochondrial DNA contained three major D-loop DNA species whose 3' ends mapped to three different sites. These sites were not found in the porcine sequence. However, the bovine termination sites were located 60 to 64 base pairs 5' from sequences which were also very similar to the termination-associated sequences present in pigs and other mammals. These results firmly establish the concept that arrest of heavy-strand DNA synthesis is an event determined, at least in part, by template sequence. They also suggest that arrest is determined by sequences which are a considerable physical distance away from the actual termination site.
