Protospacer-adjacent motif (PAM) (PAM)

The genomic DNA protospacer adjacent to a protospacer-adjacent motif (PAM) is the fundamental recognition site for CRISPR-Cas genome editing technologies. The PAM is a short, conserved DNA sequence (e.g., 5'-NGG-3' for SpCas9) located immediately adjacent to the target DNA sequence, known as the protospacer (Mojica et al., 2009, Microbiology). This motif is essential for the Cas nuclease to distinguish between 'self' DNA (the CRISPR array in the bacterial genome) and 'non-self' DNA (invading viral or plasmid DNA), as the PAM is absent in the host's own CRISPR locus (Sternberg et al., 2014, Nature). In a therapeutic context, the Cas-gRNA complex utilizes the PAM as an anchor to initiate DNA unwinding and subsequent base pairing with the protospacer, enabling precise genomic modifications such as gene disruption or correction (Doudna & Charpentier, 2014, Science). While highly effective for treating genetic disorders like sickle cell disease, the requirement for a specific PAM limits the targetable range of the genome, and unintended binding to 'PAM-like' sequences can lead to deleterious off-target effects (Jinek et al., 2012, Science). Consequently, engineering Cas variants with expanded PAM compatibility or higher fidelity is a major focus of current biotechnological research.

Other names

PAM sequenceProtospacerCRISPR target siteTarget DNA motif

The CRISPR-Cas complex (e.g., Cas9-gRNA) scans genomic DNA for a specific Protospacer-adjacent motif (PAM). Upon PAM recognition, the Cas protein facilitates DNA melting, allowing the guide RNA to hybridize with the complementary protospacer sequence, which leads to site-specific double-strand breaks or base editing (Jinek et al., 2012, Science; Sternberg et al., 2014, Nature).