Multiple independent horizontal transfers of informational genes from bacteria to plasmids and phages: implications for the origin of bacterial replication machinery

In contrast to the universality of other central genetic mechanisms, the re plication machinery of Bacteria is clearly different from those of Archaea and Eukaryotes. A large number of bacterial genes involved in DNA replicati on can also be found in plasmids and phages. Based on this, it has been rec ently proposed that the ancestral bacterial genes were displaced by non-ort hologous replication genes from plasmids and phages, which would explain th e profound difference between Bacteria and the other domains of life. The a lternative hypothesis is that these DNA replication genes have been frequen tly transferred from bacterial hosts to the genomes of their plasmids and p hages. The phylogenetic analysis of the bacterial DNA replication proteins most abundant in databases (replicative helicase DnaB, single-strand bindin g protein Ssb and topoisomerase TopB) presented here supports the latter hy pothesis. Each protein tree shows that sequences from plasmids and phages b ranch close to their bacterial-specific hosts, suggesting multiple independ ent horizontal transfers. Therefore, there is no evidence so far for non-or thologous gene displacement of these genes.