CRISPR-Cas12a REC2-Nuc interactions drive target-strand cleavage and constrain trans cleavage

CRISPR-Cas12a mediates RNA-guided cleavage of double-stranded DNA in cis, after which it remains catalytically active and non-specifically cleaves single-stranded DNA in trans. Native host defence by Cas12a employs cis cleavage, which can be repurposed for the genome editing of other organisms, and trans cleavage can be used for in vitro DNA detection. Cas12a orthologues have high structural similarity and a conserved mechanism of DNA cleavage, yet highly different efficacies when applied for genome editing or DNA detection. By comparing three well-characterized Cas12a orthologues (FnCas12a, LbCas12a, and AsCas12a), we sought to determine what drives their different cis and trans cleavage and how this relates to their applied function. We integrated in vitro DNA cleavage kinetics with molecular dynamics simulations, plasmid interference in Escherichia coli, and genome editing in human cell lines. We report large differences in cis cleavage kinetics between orthologues, which may be driven by dynamic REC2-Nuc interactions. We generated and tested REC2 and Nuc mutants, including a hitherto unstudied 'Nuc-loop', integrity of which is critical for the function of Cas12. In total, our in vitro, in vivo, and in silico survey of Cas12a orthologues highlights key properties that drive their function in biotechnology applications.