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. 2015 Oct 16;16(10):24751–24771. doi: 10.3390/ijms161024751

Figure 1.

Figure 1

Streptococcus pyogenes (Sp) Cas9 recognition of target DNA. Sp Cas9 consists of two lobes, one containing the HNH and RuvC nuclease domains, and an α helix lobe. Arginine residues (R1333 and R1335) of Sp Cas9 recognize the dinucleotide guanine bases of the “NGG” protospacer adjacent motif (PAM). Then, serine (S1109) and lysine (K1107) form a phosphate lock by interacting with a phosphate residue adjacent to the PAM site, which triggers the melting of the double stranded DNA so that sgRNA (green) can form Watson–Crick pairing with a complementary DNA strand. D10 and H840 are the main catalytic residues in the HNH and RuvC nuclease domains, respectively. Aspartic acid at 1135 (D1135) interacts with the third PAM guanine residue through a water molecule, and a modification of the residue to glutamic acid (D1135E) has been shown to improve the recognition of PAM sequence and target specificity.