Structural insight into replicative helicase loading in Escherichia coli.

Abstract

DNA replication is an essential, precisely regulated process that occurs once in a cell cycle. In the Gram-negative bacterium Escherichia coli, the replicative helicase EcDnaB and the helicase loader EcDnaC play key roles in the initiation step at the replication origin, oriC. EcDnaB and EcDnaC form a heterododecamer, in which hexameric EcDnaB is bound to hexameric EcDnaC. Using genetic, biochemical and structural biology approaches, many groups have probed the mechanism of replicative helicase loading, using helicases and helicase loaders from various species. Recent X-ray crystallography and cryogenic electron microscopy (cryo-EM) structural studies of the EcDnaB-EcDnaC complex revealed that the interaction of DnaC with DnaB triggers distortion accumulation on the closed ring of hexameric DnaB, inducing DnaB subunits to adopt the open helical form for replication progression. The high-resolution crystal structure of the DnaB-DnaC complex solved by Nagata et al. contributed to a better understanding of the conformational rearrangement of the DnaB ring. In addition to the structural alterations in DnaB subunits by DnaC, the binding of single-stranded DNA (ssDNA) substrates alters the ATP- and ADP-bound forms of DnaB and DnaC. These studies have proposed mechanisms by which DnaC regulates helicase loading onto ssDNA.