During the evolution of life on Earth, two distinct DNA replication machineries have emerged: that shared by the archaea and eukaryotes and that of bacteria. DNA replication is fundamental to the life of all cells, so this dichotomous evolutionary distribution is surprising. Here, we describe the identification of a protein with homology to eukaryotic DNA primase and MCM encoded within a prophage that is integrated in the genome of the bacterium Bacillus cereus. Despite mechanistic similarities, the core components of the bacterial and archaeal/eukaryotic DNA replication machineries possess little primary sequence homology (Edgell and Doolittle, 1997; Leipe et al., 1999;Forterre, 1999). A nonorthologous gene displacement event has been proposed to account for this dichotomy, with the original genes having been replaced by nonhomologous counterparts. In the replicon takeover hypothesis, Forterre has suggested that a viral origin for these proteins may explain this puzzling gap in the evolutionary tree (Forterre, 1999). However, to date there has been no direct evidence for bacteriophage harboring primary sequence homologs of core archaeal/eukary otic replication proteins or archaeal/eukaryal viruses with bacterial replication-associated genes. In this light,...