Direct observation of structure-function relationship in a nucleic acid–processing enzyme

Engineering superenzyme function Understanding how protein domains and subunits operate is critical for engineering novel functions into proteins. Arslanet al. introduced intramolecular crosslinks between two domains of theEscherichia coli helicase Rep, which unwinds DNA. By inserting linkers of different lengths, the domains can be held either “open” or “closed.” The closed conformation activates the helicase, but it can also generate super-helicases capable of unzipping long stretches of DNA at high speed and with considerable force. Comstocket al. used optical tweezers and fluorescence microscopy to simultaneously measure the structure and function of the bacterial helicase UvrD. They monitored its DNA winding and unwinding activity and its shape during these activities. The motor domain also has a “closed” conformation during DNA unwinding and switches to a reversed “open” conformation during the zipping-up interaction.Science , this issue p.344 and p.352