Csm3

Csm3 is a Cas7-like subunit of the Type III CRISPR-Cas effector complex, typically found in Type III-A systems. It is encoded in operons with other cas genes such as cas10 (Csm1) and additional csm components (Csm2, Csm4, Csm5). Csm3 is regarded as a backbone subunit that binds the CRISPR RNA (crRNA) and arranges along the crRNA-target duplex within the Csm complex.

Function and mechanism: Csm3 functions as an endoribonuclease that cleaves target RNA at regular intervals, producing fragments separated by roughly six nucleotides. The cleavage occurs within the portion of the target RNA paired to the crRNA and is attributed to a conserved catalytic site within Csm3. The RNase activity is metal-dependent in vitro and is essential for efficient interference by the Type III complex. Csm3 operates in concert with other Csm subunits to enable recognition and degradation of invading transcripts. In addition to RNA cleavage, binding of target RNA to the Csm-crRNA complex can activate Cas10 (Csm1)–mediated signaling, leading to amplification of the immune response through auxiliary nucleases and related pathways.

Structure and family: Csm3 belongs to the Cas7 superfamily, sharing structural motifs with other backbone nucleases that form a helical filament along crRNA. In most organisms with Type III-A systems, Csm3 is encoded within the cas gene cluster adjacent to cas10/csm1 and other csm genes, reflecting its integrated role in the effector complex.

Biological context: Csm3 is found in bacteria and archaea carrying Type III CRISPR-Cas systems. Its activity contributes to the immune defense by directly cleaving target RNA and by promoting downstream signaling that coordinates a broader antiviral response.

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