Differential Allosteric Modulation of Cas9 Specificity

Both RNA- and protein-based strategies have been developed to mitigate off-target cleavage by CRISPR-Cas9, yielding noncanonical guide RNAs (gRNAs) and Cas9 variants with enhanced gene-editing precision. However, the molecular mechanisms by which such PAM-distal alterations─remote from the nuclease centers─modulate Cas9 activity and specificity remain incompletely understood. Here, we performed <i>near</i>-<i>millisecond</i> all-atom molecular dynamics simulations to elucidate how diverse PAM-distal perturbations─including gRNA truncation, base mismatching, and evolved mutations─reshape the conformational dynamics and allosteric regulation of Cas9. Despite their distinct origins, all perturbations ultimately modulate Cas9 function by altering HNH dynamics that impede the transition from the checkpoint to the catalytically active state, yet they do so through distinct allosteric routes. The 16-nt gRNA induces a pronounced REC3 reorientation toward the L2 linker and HNH domain, while PAM-distal mismatches with the 18-nt gRNA promote engagement of the unwound target DNA strand with L2─both effectively restraining HNH rotation. In contrast, evolved mutations remodel the global motional modes so that REC2 swivels inward, constraining the HNH flexibility. These perturbations delineate multiple structural paths converging on a shared allosteric outcome─HNH immobilization and catalytic suppression─thereby unifying RNA-, DNA-, and protein-level effects within a single dynamic framework linking distal structural perturbations to activity control. This work provides mechanistic insight into the regulation of Cas9 fidelity and offers principles for the design of next-generation genome editors.