CRISPR-Cas9 is natively present in the adaptive immune systems of a multitude of bacteria and has been adapted as an effective genome engineering tool. The Cas9 effector enzyme, which is composed of a single Cas9 protein and a single-guide RNA (sgRNA), identifies and cleaves double-stranded DNA targets through a series of conformational changes that require DNA distortion and unwinding. While most studies of Cas9 specificity have focused on the DNA sequence, the role of intrinsic DNA physical properties ("DNA shape") in modulating Cas9 activity remains insufficiently defined. We previously showed that with a 16-nucleotide (-nt) truncated guide, the intrinsic DNA duplex dissociation energy at the PAM+(17-20) segment beyond the RNA-DNA hybrid tunes Cas9 cleavage rates of linear substrates. Here, we examined the impact of DNA supercoiling on Cas9 cleavage with the 16-nt truncated guide. Enzyme kinetic analysis revealed that PAM+(17-20) DNA sequences beyond the RNA/DNA hybrid preserve their effects on Cas9 cleavage in the supercoiled state. Furthermore, combining a novel asymmetric hairpin construct with a parallel-sequential kinetics model, rates for first-step nicking and second-step cleavage by Cas9 were obtained for both supercoiled and linear substrates. With both topologies, it was found that first-step nicking is clearly impacted by PAM+(17-20) DNA sequences, and the effects can be correlated with DNA unwinding, which dictates R-loop dynamics. This work expands our understanding of DNA target recognition by Cas9, and the methods developed, in particular those for analyzing the progression of Cas9-induced nicks, will aid in further in-depth mechanistic investigation.
