Steinbach, P. J. | Search bioRxiv

bioRxiv2024

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. We examined DNA dynamics related to damage recognition by Rad4 (yeast ortholog of XPC), which recognizes diverse lesions from environmental mutagens and initiates nucleotide excision repair. Using laser temperature-jump (T-jump), we measured the dynamics of DNA containing 3 base-pair mismatches recognized specifically by Rad4 in vitro. The T-jump kinetics traces measured using a cytosine-analog FRET pair, together with rigorous comparison with equilibrium measurements, enabled conformational dynamics to be revealed beyond the T-jump observation window of [~]20 {micro}s - 50 ms. AT-rich nonspecific sites (matched or mismatched) exhibited dynamics primarily within the T-jump window, albeit with some amplitude in "missing" fast (< 20 {micro}s) kinetics. The fast-kinetics amplitudes increased dramatically for specific sites, which were further distinguished by additional missing amplitude in slow (> 50 ms) kinetics at elevated temperatures. We posit that the rapid ({micro}s-ms) fluctuations help stall a diffusing protein at AT-rich/damaged sites and that the >50-ms kinetics reflect a propensity for specific DNA to adopt unwound/bent conformations that may resemble Rad4-bound structures. These studies provide compelling evidence for unusual DNA dynamics and deformability that likely govern how Rad4 senses DNA damage.

biophysics↗

bioRxiv2018

Static kinks or flexible hinges: conformational distributions of bent DNA bound to integration host factor mapped by fluorescence lifetime measurements

Gene regulation depends on proteins that bind to specific DNA sites. Such specific recognition often involves severe DNA deformations including sharp kinks. It has been unclear how rigid or flexible these protein-induced kinks are. Here, we investigated the dynamic nature of DNA in complex with integration host factor (IHF), a nucleoid-associated architectural protein known to bend one of its cognate sites (35 base pair H) into a U-turn by kinking DNA at two sites. We utilized fluorescence lifetime based FRET spectroscopy to map the distribution of bent conformations in various IHF-DNA complexes. Our results reveal a surprisingly dynamic specific complex: while 80% of the IHF-H population exhibited FRET efficiency consistent with the crystal structure, 20% exhibited FRET efficiency indicative of unbent or partially bent DNA. This conformational flexibility is modulated by sequence variations in the cognate site. In another site (H1) that lacks an A-tract of H on one side of the binding site, the population in the fully U-bent conformation decreased to 36%, as did the extent of bending. A similar decrease in the U-bent population was observed with a single base mutation in H in a consensus region on the other side. Taken together, these results provide important insights into the finely tuned interactions between IHF and its cognate sites that keep the DNA bent (or not), and yield quantitative data on the dynamic equilibrium between different DNA conformations (kinked or not kinked) that depend sensitively on DNA sequence and deformability. Notably, the difference in dynamics between IHF-H and IHF-H1 reflects the different roles of these complexes in their natural context, in the phage lambda \"intasome\" (the complex that integrates phage lambda into the E. coli chromosome).

biophysics↗