A high-throughput single-molecule platform to study DNA supercoiling effect on protein-DNA interactions.

DNA supercoiling significantly influences DNA metabolic pathways. To examine its impact on DNA-protein interactions at the single-molecule level, we developed a highly efficient and reliable protocol to modify plasmid DNA at specific sites, allowing us to label plasmids with fluorophores and biotin. We then induced physiological levels of negative or positive supercoiling in these plasmids using gyrase or reverse gyrase, respectively. By comparing supercoiled DNA with relaxed circular DNA, we assessed the effects of supercoiling on CRISPR-Cas9 and the mismatch repair protein MutS. We found that negative DNA supercoiling exacerbates off-target effects in DNA unwinding by Cas9. For MutS, we observed that both negative and positive DNA supercoiling enhance the binding interaction between MutS and a mismatched base pair but do not affect the rate of ATP-induced sliding clamp formation. These findings not only underscore the versatility of our protocol but also open new avenues for exploring the intricate dynamics of protein-DNA interactions under the influence of supercoiling.