Reinitiation of DNA Replication
This chapter highlights the current knowledge of replication restart in Escherichia coli and T4-infected E. coli. T4 uses homologous recombination to initiate most of its DNA replication. Soon after phage gene expression commences, DNA replication is initiated from internal replication origins, and the forks so generated travel toward the ends of the infecting genome. UvsW is an RNA-DNA helicase that disrupts the origin R-loop, an essential intermediate in origin replication. Many rounds of recombination-dependent DNA replication (RDR) convert the intracellular form of T4 DNA into a long concatemer. Investigations of phage T4 hotspots for marker rescue recombination provided a dramatic demonstration of the coupling between replication, DNA damage, and recombination. The hotspots were first detected as regions of the genome where genetic markers could be rescued by homologous recombination from UV-irradiated phage at an inflated frequency. The coordinated action of several proteins-PriA, PriB, PriC, DnaT, and DnaC--is required for replication reinitiation at nonorigin sequences in E. coli. PriA is the key protein in assembly of the primosome complex, the base of the scaffold that leads to the loading of DnaB. In addition to its primosome assembly activity, PriA has an ssDNA-dependent ATPase activity, which is also dependent on primosome assembly site (PAS) sites in the presence of single-strand binding protein (SSB). RecG strongly prefers forks with a single-strand gap on the leading strand; this and other results argue for a special role of RecG in the replication of damaged DNA.
