Dynamics of the PriA Helicase at Stalled DNA Replication Forks.

The DNA helicase PriA is a key protein for restarting stalled DNA replication forks in bacteria. With 3' to 5' helicase activity, PriA is important in primosome assembly. We used atomic force microscopy (AFM) and specifically employed time-lapse AFM to visualize the interaction of PriA with two DNA substrates. The results show that most of the PriA molecules are observed bound at the fork. However, PriA is capable of translocating over distances of about 400 bp. There is a preference for the long-range translocation of PriA depending on the fork type. For a fork with the nascent leading strand as single-stranded DNA (ssDNA; F4 substrate), PriA translocates preferentially on the parental arm of the fork. For the substrate F14, which contains an additional ssDNA segment between the parental and lagging arms (5 nt gap), PriA translocates on both the parental and lagging strand arms. These data suggest that transient formation of the single-stranded regions during the DNA replication can change the selection of the DNA duplex by PriA. Translocation of the helicase was directly visualized by time-lapse AFM imaging, which revealed that PriA can switch strands during translocation. These novel features of PriA shed new light on the mechanisms of PriA interaction with stalled replication forks.