(a) Model of XPD enhancement by RPA2. Schematic of XPD–DNA complex (left; side and front views), with 5 nt of ssDNA (black) bound at a regulatory secondary binding site on HD1 that contains H202. XPD can adopt one of two intrinsic states—a low (left) and high (right) processivity state—that correspond to different DNA fork conformations. Interconversion between both states occurs either spontaneously or through RPA2 binding of the 5 nt at the regulatory site. XPDH202A adopts a similar conformation (right). (b) Kinetic model of XPD processivity and the effect of RPA2. XPD interconverts between low and high states with rates k1 and k-1, and RPA2 shifts the equilibrium toward the high state. XPD can dissociate from DNA only from the low state with rate koff. Only in the high state is XPD able to unwind in excess of 25 bp, which occurs at a rate k2. Once an XPD molecule unwinds >25 bp, it is scored as being in the high-processivity state. (c) Representative traces of XPD exhibiting high-processivity unwinding. The time t>25 denotes the first time at which XPD crosses the 25 bp threshold. (d) Fraction of all XPD molecules that have reached high processivity (>25 bp) after time t for each RPA2 concentration. The curves are globally fit to the model in (a) using as parameters the rate constants k1, k-1, k2, and koff. (e) The rate of entry into the high-processivity state, k1, depends linearly on RPA2 concentration. For more details on the kinetic model, see Materials and methods.