PAM Site

PAM Site (Protospacer Adjacent Motif) is a short conserved DNA sequence located immediately adjacent to the CRISPR target that is essential for Cas nuclease recognition and cleavage ¹.

How It Works

The PAM serves as a molecular signal that distinguishes foreign DNA from the host’s own CRISPR array, where spacer sequences lack flanking PAMs. For the widely used Streptococcus pyogenes Cas9 (SpCas9), the canonical PAM is 5’-NGG-3’, positioned immediately downstream of the 20-nucleotide target on the non-target strand.

PAM recognition is the first step in target engagement. Cas9 interrogates the genome by transiently binding PAM sites; only when a PAM is present does the enzyme unwind the adjacent DNA to check for guide RNA complementarity. Without a valid PAM, Cas9 cannot initiate strand separation and the site is skipped.

The PAM requirement constrains targetable sites to approximately every 8 base pairs in the genome for SpCas9. To expand targeting range, researchers have engineered Cas9 variants with relaxed or altered PAM specificities (xCas9, SpCas9-NG, SpRY) and adopted orthogonal nucleases like Cas12a (which recognizes a T-rich PAM) for sites inaccessible to standard SpCas9 ².

Computational Considerations

Guide design software identifies all PAM-adjacent sequences in a target region and ranks them by predicted on-target efficiency and off-target specificity. PAM-aware algorithms account for the specific Cas variant being used and can suggest alternative nucleases when the preferred target site lacks a compatible PAM ².

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