Coupling DNA unwinding activity with primer synthesis in the bacteriophage T4 primosome

The unwinding and priming activities of the bacteriophage T4 primosome, which consists of a hexameric helicase (gp41) translocating 5′ to 3′ and an oligomeric primase (gp61) synthesizing primers 5′ to 3′, has been investigated on DNA hairpins manipulated by a magnetic trap. We find that the T4 primosome continuously unwinds the DNA duplex while allowing for primer synthesis through a primosome disassembly mechanism or a novel DNA looping mechanism. A fused gp61-gp41 primosome unwinds and primes DNA exclusively via the DNA looping mechanism. Other proteins within the replisome control the partitioning of these two mechanisms disfavoring primosome disassembly thereby increasing primase processivity. In contrast priming in bacteriophage T7 involves discrete pausing of the primosome and in Escherichia coli appears to be associated primarily with dissociation of the primase from the helicase. Thus nature appears to use several strategies to couple the disparate helicase and primase activities within primosomes.