The bacterial replicative helicase DnaB evolved from a RecA duplication.

The RecA/Rad51/DCM1 family of ATP-dependent recombinases plays a crucial role in genetic recombination and double-stranded DNA break repair in Archaea, Bacteria, and Eukaryota. DnaB is the replication fork helicase in all Bacteria. We show here that DnaB shares significant sequence similarity with RecA and Rad51/DMC1 and two other related families of ATPases, Sms and KaiC. The conserved region spans the entire ATP- and DNA-binding domain that consists of about 250 amino acid residues and includes 7 distinct motifs. Comparison with the three-dimensional structure of Escherichia coli RecA and phage T7 DnaB (gp4) reveals that the area of sequence conservation includes the 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 previously undetected DnaB homolog that might function in the mitochondria. The DnaB homolog from Arabidopsis also contains a DnaG-primase domain and the DnaB homolog from the nematode seems to contain an inactivated version of the primase. This domain organization is reminiscent of bacteriophage primases-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 divergence of the bacteria from Archaea and eukaryotes, which indicates that the replication fork helicases in Bacteria and Archaea/Eukaryota have evolved independently.

ATP依赖的重组酶RecA/Rad51/DCM1家族在古菌、细菌和真核生物的基因重组以及双链DNA断裂修复中起着至关重要的作用。DnaB是所有细菌中的复制叉解旋酶。我们在此表明，DnaB与RecA和Rad51/DMC1以及另外两个相关的ATP酶家族Sms和KaiC具有显著的序列相似性。保守区域跨越整个由约250个氨基酸残基组成的ATP和DNA结合结构域，包括7个不同的基序。与大肠杆菌RecA和噬菌体T7 DnaB（gp4）的三维结构进行比较后发现，序列保守区域包括中心平行的β -折叠以及大部分连接的螺旋和环，还有一个由氨基末端螺旋和羧基末端β -曲折构成的较小结构域。此外，我们发现动物、植物和疟原虫（但不包括酿酒酵母）编码一种先前未被检测到的DnaB同源物，它可能在线粒体中发挥作用。拟南芥的DnaB同源物还包含一个DnaG引发酶结构域，线虫的DnaB同源物似乎包含一个失活形式的引发酶。这种结构域组织使人联想到噬菌体引发酶 - 解旋酶，并表明DnaB可能是通过噬菌体载体水平转移到真核生物细胞核基因组中的。我们假设DnaB起源于细菌从古菌和真核生物分化之后一个类似RecA的祖先的复制，这表明细菌中的复制叉解旋酶与古菌/真核生物中的是独立进化的。