Fig. 4.

Model for target recognition and Cas3 recruitment by Cascade. (A) Cascade searches dsDNA for PAM sites. (B) CasA recognizes the PAM sequence via the L1 loop. (C) Once CasA has located a bona fide target, a rearrangement of Cascade facilitates crRNA–protospacer DNA hybridization. (D) CasA positions the displaced, non-target strand for recruitment of the transacting Cas3 nuclease. (E) Following initial nicking of the displaced strand, Cas3 loads onto the newly formed ssDNA end and translocates along the substrate during processive degradation of the target dsDNA.