Figure 4.

Comparison of type I, II and III CRISPR systems in crRNA maturation and interference. Upon transcription of CRISPR following the acquisition stage, pre-crRNA undergoes a maturation stage, which is processed by Cas6 in type I and III. In type II, the maturation step is performed by Cas9 accompanied by tracer RNA and RNase III. The interference step varies notably between the different types. CRISPR, clustered regularly interspaced short palindromic repeats; crRNA, CRISPR-associated RNA; Cas, CRISPR-associated protein; pre-crRNA, precursor crRNA; PAM, protospacer adjacent motif.