Helicase-SSB interactions in recombination-dependent DNA repair and replication

Dda, one of three helicases encoded by bacteriophage T4, has been wellcharacterized biochemically but its biological role remains unclear. It is thought to be involved in origin-dependent replication, recombination-dependent replication, antirecombination, recombination repair, as well as in replication fork progression past template-bound nucleosomes and RNA polymerase. One of the proteins that most strongly interacts with Dda, Gp32, is the only single-stranded DNA binding protein (SSB) encoded by T4, is essential for DNA replication, recombination, and repair. Previous studies have shown that Gp32 is essential for Dda stimulation of replication fork progression. Our studies show that interactions between Dda and Gp32 play a critical role in regulating replication fork restart during recombination repair. When the leading strand polymerase stalls at a site of ssDNA damage and the lagging strand machinery continues, Gp32 binds the resulting ssDNA gap ahead of the stalled leading strand polymerase. We found that a Gp32 cluster on leading strand ssDNA blocks Dda loading on the lagging strand ssDNA, blocks stimulation of fork progression by Dda, and stimulates Dda to displace the stalled polymerase and the 3’ end of the daughter strand. This unwinding generates conditions necessary for polymerase template switching in order to regress the DNA damage-stalled replication fork. Helicase trafficking by Gp32 could play a role in preventing premature fork progression until the events required for error-free translesion DNA synthesis have taken place. Interestingly, we found that Dda helicase activity is strongly stimulated by the N-terminal deletion mutant Gp32-B, suggesting the N-terminal truncation to generate Gp32-B reveals a cryptic helicase stimulatory activity of Gp32 that may be revealed in the context of a moving polymerase, or through direct interactions of Gp32 with other replisome components. Additionally, our findings support a role for Dda-Gp32 interactions in double strand break (DSB) repair by homology-directed repair (HDR), which relies on homologous recombination and the formation of a displacement loop (D-loop) that can initiate DNA synthesis. We examined the D-loop unwinding activity of Dda, Gp41, and UvsW, the D-loop strand extension activity of Gp43 polymerase, and the effect of the helicases and their modulators on D-loop extension. Dda and UvsW, but not Gp41, catalyze D-loop invading strand by DNA unwinding. The relationship between Dda and Gp43 was modulated by the presence of Gp32. Dda D-loop unwinding competes with Dloop extension by Gp43 only in the presence of Gp32, resulting in a decreased frequency of invading strand extension when all three proteins are present. These data suggest Dda functions as an antirecombinase and negatively regulates the replicative extension of Dloops. Invading strand extension is observed in the presence of Dda, indicating that invading strand extension and unwinding can occur in a coordinated manner. The result is a translocating D-loop, called “bubble migration synthesis,” a hallmark of break-induced repair (BIR) and synthesis dependent strand annealing (SDSA). Gp41 did not unwind Dloops studied and may serve as a secondary helicase loaded subsequent to D-loop processing by Dda. Dda is proposed to be a mixed function helicase that can work both as an antirecombinase and to promote recombination-dependent DNA synthesis, consistent with the notion that Dda stimulates branch migration. These results have implications on the repair of ssDNA damage, DSB repair, and replication fork regulation, which are highly conserved processes sustained in all organisms.