Cas7

Cas7 is a CRISPR-associated protein that forms a core component of the Cascade surveillance complex in many Type I CRISPR-Cas systems. It typically serves as a backbone subunit, assembling with several other Cas proteins and the CRISPR RNA (crRNA) to create a multi-subunit filament that binds to target DNA.

In Cascade assemblies, multiple Cas7 subunits arrange into a helical filament that coats the crRNA. This arrangement provides the structural scaffold for crRNA presentation and helps position the guide sequence for recognition of complementary DNA. Cas7 works in concert with other Cascade components such as Cas5 and the large subunit Cas8 (also known as Cse1 in some systems); Cas6 is involved in crRNA processing in certain contexts. Target recognition occurs when Cascade binds to a protospacer on the foreign DNA adjacent to a PAM sequence, forming an R-loop. Recruitment of the nuclease-helicase Cas3 then mediates target degradation.

Cas7 proteins are generally around 30–40 kDa per subunit and share a conserved fold that supports RNA binding along the crRNA. The number of Cas7 subunits in Cascade varies by organism and CRISPR subtype, reflecting diversity across bacteria and some archaea. The cas7 gene is commonly found in operons with other Cascade components and is distributed broadly among prokaryotes, illustrating a widespread but specific role within Type I CRISPR-Cas defense systems.

In summary, Cas7 is primarily a structural, crRNA-binding element of Cascade that contributes to target recognition, while the actual nuclease activity against invading DNA is carried out by Cas3 following Cascade activation.