Twin DNA pumps of a hexameric helicase provide power to simultaneously melt two duplexes.

DnaB is the primary replicative helicase in Escherichia coli. We show here that DnaB can unwind two duplex arms simultaneously for an extended distance provided that two protein rings are positioned on opposite sides of the duplex arms. A putative eukaryotic replication fork helicase, Mcm4,6,7, performs a similar activity. Double-ringed melting of duplexes may function at a replication fork in vivo. This mechanism may apply to RuvB, since the proteins share mechanistic similarities. Thus, two RuvB hexamers may function in coordination at a Holliday junction to overcome regions of DNA heterology and DNA lesions. Furthermore, DnaB can actively translocate along DNA while encircling three DNA strands. Therefore, if DnaB encounters a D loop during fork progression, it will encircle the invading strand and may convert the recombinative invading strand to a daughter lagging strand. Finally, we present evidence that the DNA binding site of DnaB is buried inside its central channel.