{"id":1934,"date":"2024-05-14T13:52:07","date_gmt":"2024-05-14T13:52:07","guid":{"rendered":"https:\/\/www.idtdna.com\/page\/crispr-cas-nucleases-understanding-pam-requirements-and-cas-diversification-strategies"},"modified":"2025-08-22T16:25:17","modified_gmt":"2025-08-22T16:25:17","slug":"crispr-cas-nucleases-understanding-pam-requirements-and-cas-diversification-strategies","status":"publish","type":"post","link":"https:\/\/eu.idtdna.com\/page\/support-and-education\/decoded-plus\/crispr-cas-nucleases-understanding-pam-requirements-and-cas-diversification-strategies\/","title":{"rendered":"CRISPR-Cas nucleases: Understanding PAM requirements and Cas diversification strategies"},"content":{"rendered":"

What are PAM requirements for CRISPR?<\/h2>\n

In CRISPR experiments, protospacer adjacent motifs (PAM) are short sequences of genomic DNA located directly next to the targeted modification site. PAM sequences are an important requirement for any CRISPR experiment as they are the parts of DNA that the Cas nuclease recognizes. PAM sequences act as a signal for Cas nucleases to let them know they have found the correct modification site.<\/p>\n

The Streptococcus pyogenes<\/em> Cas9 (sp<\/em>Cas9) nuclease recognizes an NGG PAM (Figure 1), other nucleases recognize different PAMs. For example, the Cas12 nuclease from Acidaminococcus <\/em>sp.<\/em> (as<\/em>Cas12a or Cpf1<\/a>) relies on a TTTV sequence (Figure 2). Cas nucleases won’t cut the PAM site itself but modify the DNA upstream or downstream of the PAM site. More specifically, Cas 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<\/a>].<\/p>\n

\"Components
Figure 1. Components of a CRISPR-Cas9 system for directing Cas9 endonuclease to genomic targets. The crRNA:tracrRNA complex uses crRNA and tracrRNA sequences that hybridize and then form a complex with Cas9 endonuclease to guide targeted cleavage of genomic DNA. The cleavage site is specified by the spacer element of the crRNA (light blue bar). The crRNA spacer element recognizes 19 or 20 nt on the strand opposite from the NGG PAM site. The PAM site must be present immediately downstream of the protospacer element for cleavage to occur.<\/figcaption><\/figure>\n<\/p>\n

\"Components
Figure 2. Components of a CRISPR-Cas12a system for directing Cas12a endonuclease to genomic targets. Cas12a (Cpf1) crRNA forms a complex with Cas12a endonuclease to guide targeted cleavage of genomic DNA. The cleavage site is specified by the spacer element of the crRNA (blue bar). The crRNA spacer element recognizes 21 nt on the opposite strand of the TTTV PAM site. The PAM site must be present immediately upstream of the protospacer element for cleavage to occur. PAM = protospacer adjacent motif; V = A, C, or G.<\/figcaption><\/figure>\n<\/p>\n

However, the targeted site in the genome may not always be downstream from a NGG or TTTV PAM sequence. Fortunately, in addition to the variety of PAMs offered by wild-type Cas nucleases isolated in different bacteria, researchers have also created modified Cas enzymes to recognize an even larger variety of PAM sequences. This provides greater flexibility when selecting modification target sites in a genome. <\/p>\n

Engineering Cas nucleases to recognize novel PAMs<\/h2>\n

One example of Cas nucleases modified to recognize different PAMs are the Alt-R™ CRISPR-Cas12a nucleases, which come in two formats:<\/p>\n