Fig. 1

The stages of CRISPR/Cas adaptive immunity. The three phases of the CRISPR/Cas9 system are depicted schematically. When phage DNA is injected into a bacterial cell, the Cas1–Cas2 adaptation module proteins are activated, which remove spacer-sized segments of phage DNA and channel them into the CRISPR array. The CRISPR array is transcribed, and the resulting pre-crRNA is processed at repeat sequences to form crRNAs during CRISPR RNA biogenesis. The Cas protein effectors bind individual crRNAs. Effectors programmed by suitable crRNA attach to phage DNA with sequences matching a CRISPR spacer in the cell, and the resulting R-loop complex is destroyed by Cas executor nuclease