STRAND SWITCHING OF A REPLICATIVE DNA HELICASE PROMOTED BY THE ESCHERICHIA-COLI PRIMOSOME

The E. coli primosome assembles at an origin on a single-stranded DNA, like that of PHIX174, to promote replication of that template. Upon c onversion to the duplex form, the primosome can generate a rolling cir cle product from this template. Rolling circle synthesis implies the t ransfer of the DnaB helicase from its initial loading site on the vira l strand to a displaced complementary strand. Isolated primosomes prom ote only unit-length synthesis; supplementation with PriC, DnaC, and D naT is necessary to reconstitute rolling circle synthesis. Rolling cir cle replication is sensitive to salts, whereas primosome assembly and unit-length synthesis are not. Thus, the primosome promotes two distin ct reactions: assembly for first-round synthesis and strand switching for rolling circle synthesis.