Evidence for intrinsic DNA dynamics and deformability in damage sensing by the Rad4/XPC nucleotide excision repair complex.

Altered DNA dynamics at lesion sites are implicated in how DNA repair proteins sense damage within genomic DNA. Using laser temperature-jump (T-jump) spectroscopy combined with cytosine-analog Förster Resonance Energy Transfer (FRET) probes that sense local DNA conformations, we measured the intrinsic dynamics of DNA containing 3 base-pair mismatches recognized in vitro by Rad4 (yeast ortholog of XPC). Rad4/XPC recognizes diverse lesions from environmental mutagens and initiates nucleotide excision repair. T-jump measurements, together with a novel and rigorous comparison with equilibrium FRET, uncovered conformational dynamics spanning multiple timescales and revealed key differences between Rad4-specific and non-specific DNA. AT-rich non-specific sites (matched or mismatched) exhibited dynamics primarily within the T-jump observation window, albeit with some amplitude in 'missing' fast (<20 μs) kinetics. These fast-kinetics amplitudes were dramatically larger for specific sites (CCC/CCC and TTT/TTT), which also exhibited 'missing' slow (>50 ms) kinetics at elevated temperatures, unseen in non-specific sites. We posit that the rapid (μs-ms) intrinsic DNA fluctuations help stall a diffusing protein at AT-rich/damaged sites and that the >50-ms kinetics in specific DNA reflect a propensity to adopt unwound/bent conformations resembling Rad4-bound DNA structures. These studies provide compelling evidence for sequence/structure-dependent intrinsic DNA dynamics and deformability that likely govern damage sensing by Rad4.