Split Cas12a Protospacer Engineering Enables Ultraspecific Recognition of a PAM-Less DNA Strand via an External PAM-Proximal Duplex.

CRISPR-Cas12a is a programmable RNA-guided endonuclease whose activation by double-stranded DNA normally depends on the recognition of an adjacent protospacer-adjacent motif (PAM), which constrains target selection in diagnostic applications. Here, we divide the DNA activator into PAM-proximal (Pp) and PAM-distal (Pd) duplexes, spatially separating the PAM requirement from the interrogation of a PAM-less target sequence. FRET, electrophoretic mobility shift, fluorescence polarization, and functional assays indicate that either fragment can associate with Cas12a, but that robust productive activation requires the PAM-containing Pp duplex together with the Pd duplex. Our data further support a model in which PAM-mediated protein-DNA contacts stabilize an assembly-competent complex and facilitate binding of the PAM-less Pd target capture. Weakening hybridization between the guide and Pp with a chimeric DNA/RNA increased the dependence on PAM-mediated stabilization and improved structural selectivity in the Pp region and single-nucleotide specificity in the Pd region. The resulting split-duplex architecture provides an ultraspecific platform for resolving single-nucleotide polymorphisms in the PAM-less DNA strand.