Expanding the targeting scope of FokI-dCas nuclease systems with SpRY and Mb2Cas12a.

CRISPR-Cas9 and Cas12a are widely used sequence-specific nucleases (SSNs) for genome editing. The nuclease domains of Cas proteins can induce DNA double strand breaks upon RNA guided DNA targeting. Zinc finger nucleases (ZFNs) and Transcription Activator-Like Effector Nucleases (TALENs) have been popular SSNs prior to CRISPR. Both ZFNs and TALENs are based on reconstitution of two monomers with each consisting of a DNA binding domain and a FokI nuclease domain. Inspired by the configuration of ZFNs and TALENs, dimeric FokI-dCas9 systems were previously demonstrated in human cells. Such configuration, based on a pair of guide RNAs (gRNAs), offers great improvement on targeting specificity. To expand the targeting scope of dimeric FokI-dCas systems, the PAM (protospacer adjacent motif)-less SpRY Cas9 variant and the PAM-relaxed Mb2Cas12a system were explored. Rice cells showed that FokI-dSpRY had more robust editing efficiency than a paired SpRY nickase system. Furthermore, a dimeric FokI-dMb2Cas12a system was developed that displayed comparable editing activity to Mb2Cas12a nuclease in rice cells. Finally, a single-chain FokI-FokI-dMb2Cas12a system was developed that cuts DNA outside its targeting sequence, which could be useful for many versatile applications. Together, this work greatly expanded the FokI based CRISPR-Cas systems for genome editing.