Sequence Engineering of Guide DNA for Precise RNA Targeting by Cas12a.
Cas12a is highly accommodative toward noncanonical activation pathways to the extent of flipping its identity to be a DNA-guided RNA-targeting effector. A sequence engineering approach was used to systematically identify desirable guide DNA (gDNA) sequence motifs to achieve comparable RNA targeting efficiency as the canonical RNA-guided Cas12a with good selectivity down to single-nucleotide mismatch. Importantly, we introduced a split gDNA design concept with greater energetic differences arising from subtle nucleotide changes to probe the key spacer features for effective Cas12a-gDNA activation. Similar to the canonical RNA-guided activation pathway, Cas12a was found to engage actively in the "seed-like" scaffold-proximal region while the scaffold-distal region was largely hybridization-driven. We further evolved the split gDNA design to enhance the sequence selectivity by up to 21-fold compared to a single gDNA design and achieve single-nucleotide discrimination among representative let-7 family members. This study has established a gDNA sequence design framework to reprogram Cas12a as a precise RNA targeting platform.