CRISPR/Cas9 System: A Bacterial Tailor for Genomic Engineering

Figure 1

An overview of type II CRISPR/Cas immunity. The CRISPR/Cas system provides the adaptive immunity to prokaryotes against the foreign DNA in three phases. (A) Adaptation: during the adaptation phase, the Cas1-Cas2 complex selects the new spacer (red) and integrates it into the leader-proximal end of CRISPR locus. (B) crRNA biogenesis: in this phase, the CRISPR locus is transcribed into pre-crRNA that forms duplexes with tracr-RNAs with repeat-anti-repeat interaction followed by recognition and cleavage by RNase III into mature crRNA in the presence of Cas9. (C) Interference: during this phase, the mature crRNA/tracr RNA hybrid that remains bound to Cas9 acts as a guide for Cas9 to recognize and degrade the foreign DNA upon subsequent infection.