Molecular basis for dual-spacer-guided target cleavage by the TIGR-TasH system

The RNA-directed programmable nuclease systems, exemplified by the CRISPR-Cas system, have been widely used in genome editing. In contrast to the single-spacer configuration of CRISPR RNA (crRNA), the guide RNA (tigRNA) of the tandem interspaced guide RNA (TIGR) system features a dual-spacer arrangement, thereby directing the TIGR-associated (Tas) protein to engage both strands of the target double-stranded DNA (dsDNA). Here, we determine six cryo-electron microscopy structures of the Salicola phage TIGR-TasH complex. The central coiled-coil region of TasH mediates dimerization, while the C-terminal nucleolar protein (Nop) domain is able to autonomously process precursor tigRNA. Upon target binding, the dynamic N-terminal HNH nuclease domain is recruited for cleavage through a β-hairpin, which also determines the target preference. More interestingly, the conserved box C motif of tigRNA stabilizes this β-hairpin in an adenine-specific manner, enabling us to rationally design a guide RNA-defined nickase, distinct from conventional protein-based nickase strategies used in genome editing.