Host factors that promote transpososome disassembly and the PriA-PriC pathway for restart primosome assembly

Initiation of bacteriophage Mu DNA replication by transposition requires the disassembly of the transpososome that catalyses strand exchange and the assembly of a replisome promoted by PriA, PriB, PriC and DnaT proteins, which function in the host to restart stalled replication forks. Once the molecular chaperone ClpX weakens the very tight binding of the transpososome to the Mu ends, host disassembly factors (MRF alpha-DF) promote the dissociation of the transpososome from the DNA template and the assembly of a new nucleoprotein complex. Prereplisome factors (MRF alpha-PR) further alter the complex, allowing PriA binding and loading of major replicative helicase DnaB onto the template promoted by the restart proteins. MRF alpha-PR is essential for DnaB loading by restart proteins even on the deproteinized Mu fork whereas MRF alpha-DF is not required on the deproteinized template. When the transition from transpososome to replisome was reconstituted using MRF alpha-DF and MRF alpha-PR, initiation of Mu DNA replication was strictly dependent upon added PriC and PriA helicase. In contrast, initiation on the deproteinized template was predominantly dependent upon PriB and did not require PriA's helicase activity. The results indicate that transition mechanisms beginning with the transpososome disassembly can determine the pathway of replisome assembly by restart proteins.