Nanopore tweezers measurements of RecQ conformational changes reveal the energy landscape of helicase motion.

ABSTRACT

Helicases are essential for nearly all nucleic acid processes across the tree of life, yet detailed understanding of how they couple ATP hydrolysis to translocation and unwinding remains incomplete because their small (∼300 picometer), fast (∼1 ms) steps are difficult to resolve. Here, we use Nanopore Tweezers to observe single Escherichia coli RecQ helicases as they translocate on and unwind DNA at ultrahigh spatiotemporal resolution. Nanopore Tweezers simultaneously resolve individual steps of RecQ along the DNA and conformational changes of the helicase associated with stepping. Our data reveal the mechanochemical coupling between physical domain motions and chemical reactions that together produce directed motion of the helicase along DNA. Nanopore Tweezers measurements are performed under either assisting or opposing force applied directly on RecQ, shedding light on how RecQ responds to such forces in vivo. Determining the rates of translocation and physical conformational changes under a wide range of assisting and opposing forces reveals the underlying dynamic energy landscape that drives RecQ motion. We show that RecQ has a highly asymmetric energy landscape that enables RecQ to maintain velocity when encountering molecular roadblocks such as bound proteins and DNA secondary structures. This energy landscape also provides a mechanistic basis making RecQ an 'active helicase,' capable of unwinding dsDNA as fast as it translocates on ssDNA. Such an energy landscape may be a general strategy for molecular motors to maintain consistent velocity despite opposing loads or roadblocks.