Figure 2.

Overview of the CRISPR–Cas9 immune system. Adaptive immunity by CRISPR–Cas systems is mediated by CRISPR RNAs (crRNAs) and Cas proteins, which form multicomponent CRISPR ribonucleoprotein (crRNP) complexes. Three processes underlie the Cas9 system: acquisition, expression, and interference. During acquisition, foreign DNA (red) is incorporated into the CRISPR locus. Expression involves transcribing target DNA into non-coding pre-crRNAs to which trans-activating crRNAs (tracrRNAs) attach, which function as a scaffold for Cas9 binding. During interference, the Cas9 endonuclease uses these sequences to target foreign DNA for cleavage. The components of these processes are indicated based on the involved genes. The non-cas components are presented in grey, while the cas components are colored according to function: spacer acquisition (yellow); crRNA processing (green); crRNP assembly (blue); and target degradation (purple) (18).