Types
Figure 3: Three different Types of CRISPR-Cas system; the CRISPR/Cas systems have been classified into three distinct types: A) type I,B) type II and C) type III. While types I and III are found in both bacteria and archaea, type II is unique to only bacteria. The bacterial type II CRISPR/Cas system is the most studied and best characterized of the three types in which Cas9 protein is the critical component. In this system, CRISPR loci are transcribed as a precursor CRISPR RNA (pre-crRNA) containing the full set of CRISPR repeats and embedded invader-derived sequences 47. A trans-activating crRNA (tracrRNA) binds to the repeat sequences of the pre-crRNA to form a duplex RNA, which is then cleaved by a double-stranded RNA-specific ribonuclease RNase III, the Cas9 protein.