A New Role for Translation Initiation Factor 2 in Maintaining Genome Integrity
Figure 1
Transition from transpososome to replisome during bacteriophage Mu transposition.
The model reflects changes in nucleoprotein complexes at the Mu ends as the transpososome, assembled from MuA protomers, is sequentially remodeled to a replisome [5], [9], [15]. A) A supercoiled plasmid bearing a miniature version of the Mu genome serves as the donor for transposition in vitro; a second plasmid is used as the target for transposition. B) The phage-encoded MuA is assembled into an oligomeric transpososome, tightly bound to the Mu ends, and this transpososome-DNA complex is preserved as it catalyzes the transfer of Mu ends to target DNA, forming a DNA fork at each Mu end (Strand Transfer Complex 1 or STC1). The half arrows depict the 3′-OH ends of DNA at each fork, which is a potential site for initiation of Mu DNA replication. C) ClpX remodels the transpososome (STC2), weakening its interaction with DNA [14], [15] and preparing the complex for disassembly. D) An unknown host factor (MRFα-DF) completes transpososome disassembly, forming a new nucleoprotein assembly that still does not permit access of the Mu fork to replication and restart proteins. E) IF2 binds to Mu DNA and unlocks the replication block at one or both forks. F) PriA binds to the Mu fork, and its helicase action promotes the disassembly of IF2, leading to the loading of the major replicative helicase DnaB from the DnaB-DnaC complex for replisome assembly. The indicated movement of PriA on the lagging strand template may serve the dual function of promoting IF2 disassembly and unwinding the DNA helix for DnaB loading [5], [29].