TY - JOUR TI - Sequence-dependent base pair stepping dynamics in XPD helicase unwinding AU - Qi, Zhi AU - Pugh, Robert A AU - Spies, Maria AU - Chemla, Yann R A2 - Joshua-Tor, Leemor VL - 2 PY - 2013 DA - 2013/05/28 SP - e00334 C1 - eLife 2013;2:e00334 DO - 10.7554/eLife.00334 UR - https://doi.org/10.7554/eLife.00334 AB - Helicases couple the chemical energy of ATP hydrolysis to directional translocation along nucleic acids and transient duplex separation. Understanding helicase mechanism requires that the basic physicochemical process of base pair separation be understood. This necessitates monitoring helicase activity directly, at high spatio-temporal resolution. Using optical tweezers with single base pair (bp) resolution, we analyzed DNA unwinding by XPD helicase, a Superfamily 2 (SF2) DNA helicase involved in DNA repair and transcription initiation. We show that monomeric XPD unwinds duplex DNA in 1-bp steps, yet exhibits frequent backsteps and undergoes conformational transitions manifested in 5-bp backward and forward steps. Quantifying the sequence dependence of XPD stepping dynamics with near base pair resolution, we provide the strongest and most direct evidence thus far that forward, single-base pair stepping of a helicase utilizes the spontaneous opening of the duplex. The proposed unwinding mechanism may be a universal feature of DNA helicases that move along DNA phosphodiester backbones. KW - helicase KW - Xeroderma pigmentosum group D helicase KW - molecular motor KW - DNA repair KW - optical tweezer KW - single molecule JF - eLife SN - 2050-084X PB - eLife Sciences Publications, Ltd ER -