Figure 6.

A possible model for the spacer integration process in H. hispanica. DNA motifs in the middle of the first repeat are recognized by the integrase complex (consisting of the protospacer, Cas1, Cas2 and perhaps Cas4) during spacer integration. The critical AACCC motif and the sequences spanning the leader-repeat junction require a specific distance (∼10 bp) between them, possibly to together provide a docking site for the integrase complex and to precisely direct the nucleophilic attack (indicated by a) at the junction site. In contrast, the leader-distal attack (indicated by b) is non-sequence-specific, and depends on a ∼10-bp molecular ruler starting from the GTGGG motif. Note that putative important interactions between the integrase complex (or other facilitative proteins) and upstream leader sections are not depicted.