(a) Cartoon of the UvrA dimer showing the ATPase mutations used. (b) Fraction of immobile UvrA-PAmCherry mutants before and after exposure to ultraviolet light (UV). See Supplementary Fig. 6 for fitted D* distributions. UvrA mutants were expressed from a plasmid in ΔuvrA, Δmfd cells. As a control, WT UvrA-PAmCherry expressed from a plasmid was used. Errors represent s.e.m. of three experimental repeats. (c) Fraction of immobile UvrB-PAmCherry molecules recruited to DNA after exposure to UV light. Unlabelled UvrA or UvrA mutants were expressed from a plasmid in ΔuvrA. See Supplementary Fig. 7 for fitted D* distributions. Errors represent s.e.m. of three experimental repeats. (d) Cartoon showing the proposed model for the initiation of NER. Dimeric UvrA, with ATP bound in the distal site, scans the genome for lesions making transient interactions with DNA. At putative lesions (represented as a kink), UvrA performs a damage verification step lasting ∼3 s. After positive damage identification, UvrA hydrolyses ATP in the proximal site to recruit either one or two UvrB molecules to the lesion. Subsequent hydrolysis of the distal site ATP facilitates the release of UvrA from the pre-incision complex.