Single-Stranded DNA Translocation of E. coli UvrD Monomer Is Tightly Coupled to ATP Hydrolysis

Abstract

Escherichia coli UvrD is an SF1A (superfamily 1 type A) helicase/translocase that functions in several DNA repair pathways. A UvrD monomer is a rapid and processive single-stranded DNA (ssDNA) translocase but is unable to unwind DNA processively in vitro. Based on data at saturating ATP (500 μM), we proposed a nonuniform stepping mechanism in which a UvrD monomer translocates with biased (3′ to 5′) directionality while hydrolyzing 1 ATP per DNA base translocated, but with a kinetic step size of 4–5 nt/step, suggesting that a pause occurs every 4–5 nt translocated. To further test this mechanism, we examined UvrD translocation over a range of lower ATP concentrations (10–500 μM ATP), using transient kinetic approaches. We find a constant ATP coupling stoichiometry of ∼1 ATP/DNA base translocated even at the lowest ATP concentration examined (10 μM), indicating that ATP hydrolysis is tightly coupled to forward translocation of a UvrD monomer along ssDNA with little slippage or futile ATP hydrolysis during translocation. The translocation kinetic step size remains constant at 4–5 nt/step down to 50 μM ATP but increases to ∼7 nt/step at 10 μM ATP. These results suggest that UvrD pauses more frequently during translocation at low ATP but with little futile ATP hydrolysis.