The bacterial replicative helicase DnaB evolved from a RecA duplication

The RecA/Rad51/DCM1 family of ATP-dependent recombinases plays a crucial ro le in genetic recombination and double-stranded DNA break repair in Archaea , Bacteria, and Eukaryota. DnaB is the replication fork helicase in all Bac teria. We show here that DnaB shares significant sequence similarity with R ecA and Rad51/DMCI and two other related families of ATPases, Sms and KaiC. The conserved region spans the entire ATP- and DNA-binding domain that con sists of about 250 amino acid residues and includes 7 distinct motifs. Comp arison with the three-dimensional structure of Escherichia coli RecA and ph age T7 DnaB (gp4) reveals that the area of sequence conservation includes t he central parallel beta-sheet and most of the connecting helices and loops as well as a smaller domain that consists of a amino-terminal helix and a carboxy-terminal beta-meander. Additionally, we show that animals, plants, and the malarial Plasmodium but not Saccharomyces cerevisiae encode a previ ously undetected DnaB homolog that might function in the mitochondria. The DnaB homolog from Arabidopsis also contains a DnaG-primase domain and the D naB homolog from the nematode seems to contain an inactivated version of th e primase. This domain organization is reminiscent of bacteriophage primase s-helicases and suggests that DnaB might have been horizontally introduced into the nuclear eukaryotic genome via a phage vector. We hypothesize that DnaB originated from a duplication of a RecA-like ancestor after the diverg ence of the bacteria from Archaea and eukaryotes, which indicates that the replication fork helicases in Bacteria and Archaea/Eukaryota have evolved i ndependently.