Fig. 1

CRISPR-Cas systems and adaptive immunity. CRISPR repeats, together with CRISPR spacers, constitute repeat-spacer arrays that define clustered regularly interspaced short palindromic repeats (CRISPRs). These CRISPR arrays are typically flanked by CRISPR-associated sequences (cas) that encode Cas proteins involved in the three stages of CRISPR-encoded immunity, namely adaptation, expression and interference. During adaptation, Cas proteins (including the universal Cas1 and Cas2) sample invasive DNA, leading to the genesis of a new repeat-spacer unit that is inserted in a polarized manner in the CRISPR array. During the second stage — expression — the CRISPR array is transcribed into a full pre-crRNA transcript that is processed into small, mature, interfering CRISPR RNAs (crRNAs). In the third — interference — stage, crRNAs guide Cas effector proteins towards complementary nucleic acids for sequence-specific targeting. Interaction between the interference complex and the target nucleic acid is typically initiated by binding to the protospacer adjacent motif (PAM), which triggers interrogation of flanking DNA by the loaded crRNA. If complementarity extends beyond the seed sequence, an R-loop is formed, and nickase domains within Cas effector proteins cleave the target DNA. dsDNA double-stranded DNA, L leader