The YoaA-χ helicase modulates the dynamics of single-stranded DNA binding protein on DNA

The Escherichia coli helicase, YoaA, and DNA polymerase III subunit, χ, form a complex (YoaA-χ) that promotes tolerance to the DNA chain-terminator 3'-azidothymidine (AZT). Single-stranded DNA binding protein (SSB), which accumulates at stalled replication forks, also contributes to AZT tolerance through interactions with χ. Here we show that in vitro, χ mediates interactions between YoaA and SSB that modulate helicase activity in a substrate-specific manner with little effect on overhang DNA but inhibiting unwinding of forked DNA. SSB similarly affects the activity of the YoaA paralog, DinG. Single-molecule experiments show that SSB translocates with YoaA-χ, increasing both the lifetime and frequency of SSB binding events. Mutational analyses show that χ binds at the back of YoaA relative to the direction of translocation supporting a model in which YoaA-χ pulls SSB along DNA as it translocates. To our knowledge, this is the first demonstration of a mechanoenzyme pulling SSB along ssDNA.