Figure 5.

Origin of the class 2 CRISPR-Cas effectors from MGE. The figure depicts a hypothetical scenario of the origin of class 2 CRISPR-Cas from non-autonomous transposons, for type II and type V systems, and from a defence system (toxin–antitoxin module) for type VI systems. IscB and TnpB are the inferred ancestors of the type II (Cas9) and type V (Cas12) effectors, respectively. Inserts that could have contributed to increased specificity and efficiency of the effectors are shown by grey rectangles of variable size. I, II and III are the distinct amino acid motifs that jointly compose the catalytic site of the RuvC-like nuclease. TR, terminal repeats. (Online version in colour.)