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. 2013 Jul 12;14(7):14518–14531. doi: 10.3390/ijms140714518

Figure 1.

Figure 1

The interference step in the three clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated (Cas) types. Indicated for all three systems are the targeted DNA region (blue), the targeting crRNA (red) and the protospacer adjacent motif (PAM, green). In type I systems (A), the invading DNA is recognized by the Cascade:crRNA complex. The PAM motif promotes the identification of the foreign DNA. Subsequently, the nuclease, Cas3, is recruited and degrades the target DNA. Type II systems (B) require only Cas9 for interference and do not rely on a multi-protein complex. Complex formation of Cas9, tracrRNA and pre-crRNA enables RNase III to mature crRNAs. The resulting complex of Cas9 and the tracrRNA:crRNA duplex recognizes the invading nucleic acid, and the Cas9 nuclease generates blunt-ended cleavage of both DNA strands. In type III systems (C), a multi-protein complex (Csm or Cmr) or Cas6 processes pre-crRNA into mature crRNA. The complex-bound crRNA recognizes invading DNA (Csm) or RNA (Cmr), resulting in target degradation.