Stop‐action movie of UvrD helicase unwinding DNA

Abstract

Molecular mechano‐chemical coupling is a fundamental process in biology. To capture molecular locomotion by X‐ray crystallography, we determined a dozen crystal structures of UvrD‐DNA complexes in the absence or presence of ATP hydrolysis analogs. After combining multiple structures representing each functional state and accounting for crystal lattice effects, we obtained a stop‐action movie of UvrD helicase unwinding DNA one base pair per ATP hydrolyzed. For the first time, we show that each ATP‐hydrolysis cycle delivers a power stroke in two parts. Binding of ATP is coupled with unwinding of one base pair, and release of ADP and Pi is coupled with translocation of the newly unpaired single base. Combining our new mutagenesis, structural and kinetic studies with published data, we have put forward the model of dual active states of UvrD for its dual functions (dsDNA unwinding and RecA removal from ssDNA) in DNA replication and repair. The structural and functional duality is most probably essential for inhibition of unwanted helicase activity.