Assembly of the primosome of DNA replication in Escherichia coli.

Assembly of the Escherichia coli primosome requires six proteins, PriA, PriB, PriC, DnaB, DnaC, and DnaT, acting at a primosome assembly site (pas) on an SSB-coated single-stranded (ss) DNA. Assembly is initiated by interactions of PriA and PriB with ssDNA and the pas. PriC, DnaB, DnaC, and DnaT then act on the PriA-PriB-DNA complex to yield the primosome. In the primosome, the dATPase (ATPase) of PriA becomes hyper-activated. In addition, the assembled primosome appears to block the pas, preventing it from activating additional PriA molecules. Either ATP alone or dATP in combination with GTP is sufficient for primosome assembly, while ATP or GTP provides for its maintenance during isolation. These nucleotide requirements can be reconciled with the need for ATP or dATP for DnaB-DnaC complex formation and hydrolysis of ATP or GTP by DnaB when it binds ssDNA. Such isolated primosomes contain a dATPase, the hallmark of PriA, and a GTPase indicative of DnaB. Further studies indicate that the isolated primosome contains the PriB replication activity in addition to PriA and DnaB.