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.