DNA replication and strand asymmetry in prokaryotic and mitochondrial genomes.

Different patterns of strand asymmetry have been documented in a variety of prokaryotic genomes as well as mitochondrial genomes. Because different replication mechanisms often lead to different patterns of strand asymmetry, much can be learned of replication mechanisms by examining strand asymmetry. Here I summarize the diverse patterns of strand asymmetry among different taxonomic groups to suggest that (1) the single-origin replication may not be universal among bacterial species as the endosymbionts Wigglesworthia glossinidia, Wolbachia species, cyanobacterium Synechocystis 6803 and Mycoplasma pulmonis genomes all exhibit strand asymmetry patterns consistent with the multiple origins of replication, (2) different replication origins in some archaeal genomes leave quite different patterns of strand asymmetry, suggesting that different replication origins in the same genome may be differentially used, (3) mitochondrial genomes from representative vertebrate species share one strand asymmetry pattern consistent with the strand-displacement replication documented in mammalian mtDNA, suggesting that the mtDNA replication mechanism in mammals may be shared among all vertebrate species, and (4) mitochondrial genomes from primitive forms of metazoans such as the sponge and hydra (representing Porifera and Cnidaria, respectively), as well as those from plants, have strand asymmetry patterns similar to single-origin or multi-origin replications observed in prokaryotes and are drastically different from mitochondrial genomes from other metazoans. This may explain why sponge and hydra mitochondrial genomes, as well as plant mitochondrial genomes, evolves much slower than those from other metazoans.