Molecular basis of single-mismatch-induced nuclease-to-nickase conversion in TIGR-TasH

Tandem interspaced guide RNA (TIGR)-Tas systems are a distinct class of RNA-guided double-stranded DNA nucleases that employ dual-spacer guide RNAs (tigRNAs) for PAM-independent target recognition. A single mismatch between the tigRNA and target DNA can convert Salicola phage CGphi29 (Sp)TasH from a double-strand nuclease into a nickase in a position-dependent manner, but the molecular basis underlying this functional switch remains unknown. Here, we combined biochemical analyses and cryo-electron microscopy to investigate tigRNA maturation and mismatched target recognition by the Nop domain of SpTasH. We show that the Nop domain is required for pre-tigRNA processing and stabilizes the mature tigRNA through extensive interactions, thereby establishing a cleavage-competent ribonucleoprotein complex. Structural analyses of SpTasH complexes bound to substrates containing single mismatches reveal that a mismatch at the 5'-most position of spacer A is readily accommodated through Nop domain-mediated stabilization of the spacer-target heteroduplex. In contrast, a mismatch proximal to the cleavage site destabilizes the heteroduplex, preventing recruitment of the corresponding HNH domain, thereby converting the complex into a nickase. Together, these findings establish the structural basis for position-dependent mismatch recognition and reveal how Nop domain-mediated tigRNA-target stabilization enables differential responses to mismatches, providing a foundation for engineering TIGR-Tas systems for genome-editing applications.