Csm5

Csm5 is a CRISPR-associated protein that forms part of the type III-A Csm effector complex found in certain bacteria and archaea. The Csm complex is a multi-subunit ribonucleoprotein assembly that uses a CRISPR-derived guide RNA to recognize complementary RNA targets. Csm5 is one of several small, non-enzymatic subunits that accompany the catalytic Cas10-containing subunit (Csm1) and other components such as Csm2–Csm4 and Csm6 in many type III systems.

Function: The role of Csm5 is primarily structural and assembly-related. It is not thought to carry nuclease activity itself, but it contributes to the proper assembly, stabilization, and possibly positioning of the crRNA within the complex. In some organisms, disruption of Csm5 reduces interference efficiency, indicating it supports the overall function of the Csm effector complex.

Distribution and evolution: Csm5 homologs are encoded within CRISPR-Cas loci that implement type III-A interference and are found in diverse bacteria, including species of Staphylococcus and other Gram-positive bacteria, as well as some archaea that harbor type III systems. Sequence variation exists among lineages, but the subunit architecture of Csm complexes tends to be conserved.

Mechanism and signaling: Type III systems recognize target RNA guided by crRNA, triggering interference actions mediated by Cas10's activities. Csm5's contribution is non-catalytic and related to complex assembly and stability; it does not mediate target RNA cleavage directly. The exact mechanistic details, including interactions with other subunits, remain an area of active research.

Note: Csm5 is primarily of interest in basic CRISPR biology and has not been widely adopted for genome editing applications.