Single-stranded DNA binding protein hitches a ride with the Escherichia coli YoaA-χ helicase

The Escherichia coli XPD/Rad3-like helicase, YoaA, and DNA polymerase III subunit, χ, are involved in E. coli DNA damage tolerance and repair. YoaA and χ promote tolerance to the DNA chain-terminator, 3 -azidothymidine (AZT), and together form the functional helicase complex, YoaA-χ. How YoaA-χ contributes to DNA damage tolerance is not well understood. E. coli single-stranded DNA binding protein (SSB) accumulates at stalled replication forks, and the SSB-χ interaction is required to promote AZT tolerance via an unknown mechanism. YoaA-χ and SSB interactions were investigated in vitro to better understand this DNA damage tolerance mechanism, and we discovered YoaA-χ and SSB have a functional interaction. SSB confers a substrate-specific effect on the helicase activity of YoaA-χ, barely affecting YoaA-χ on an overhang DNA substrate but inhibiting YoaA-χ on forked DNA. A paralog helicase, DinG, unwinds SSB-bound DNA in a similar manner to YoaA-χ on the substrates tested. Through use of ensemble experiments, we believe SSB binds behind YoaA-χ relative to the DNA ds/ss junction and show via single-molecule assays that SSB translocates along ssDNA with YoaA-χ. This is, to our knowledge, the first demonstration of a mechanoenzyme pulling SSB along ssDNA.