CRISPR-Cas9 mechanisms recognize DNA targets that are complementary to a short CRISPR RNA (crRNA) sequence. The part of the crRNA sequence that is complementary to the target sequence is known as a spacer. In order for Cas9 to function, it also requires a specific protospacer adjacent motif (PAM) that varies depending on the bacterial species of the Cas9 gene. The most commonly used Cas9 nuclease, derived from S. pyogenes, recognizes a PAM sequence of NGG that is found directly downstream of the target sequence in the genomic DNA, on the non-target strand.
Recognition of the PAM by the Cas9 nuclease is thought to destabilize the adjacent sequence, allowing interrogation of the sequence by the crRNA, and resulting in RNA-DNA pairing when a matching sequence is present [1,2]. Cas9 nucleases with alternative PAMs have also been characterized and successfully used for genome editing . It is important to note that the PAM is not present in the crRNA sequence but needs to be immediately downstream of the target site in the genomic DNA.