Figure 8.

A schematic model representing the possible scenarios leading to Cas3/I-C mediated target DNA cleavage. Cascade/I-C (shown in blue) through base pairing between crRNA (indicated as stem and loop structure in green) and target DNA locates the invading foreign genetic element, thereby forming an R-loop. The ensuing conformational change in the large subunit of Cascade/I-C facilitates its binding to Cas3/I-C. This is followed by nucleotide independent single-stranded nick on the non-target strand by HD domain. This promotes the loading of the helicase domain of Cas3/I-C onto the ssDNA of the non-target strand. Based on the longevity of Cascade/I-C–Cas3/I-C interaction, we propose that Cas3/I-C remains attached to Cascade/I-C as a stable complex and reels in the non-target strand (this leads to DNA looping on target strand), utilizing energy from ATP. Subsequently, Cas3/I-C may hit a roadblock (e.g. a DNA-binding protein), which can trigger further DNA cleavage. Here, the cleavage site is likely to be in proximity to the Cascade/I-C.