Clustered, regularly interspaced, short palindromic repeat (CRISPR) loci play a pivotal role in the prokaryotic host defense system against invading genetic materials. The CRISPR loci are transcribed to produce CRISPR RNAs (crRNAs), which form interf ...
Clustered, regularly interspaced, short palindromic repeat (CRISPR) loci play a pivotal role in the prokaryotic host defense system against invading genetic materials. The CRISPR loci are transcribed to produce CRISPR RNAs (crRNAs), which form interference complexes with CRISPR-associated (Cas) proteins to target the invading nucleic acid for degradation. The interference complex of the type III-A CRISPR-Cas system is composed of five Cas proteins (Csm1-Csm5) and a crRNA, and targets invading DNA. This entry represents the C-terminal domain found in Csm4. Csm4 structurally resembles Cmr3, a component of the type III-B CRISPR-Cas interference complex. Studies indicate that Csm3-Csm4 complex binds single-stranded RNA in a non-sequence-specific manner. Structural analysis show , Csm3 and Csm4 have one and two ferredoxin-like folds (also known as an RRM-like fold), respectively. The long beta-hairpin inserted into the C-terminal ferredoxin-like fold of Csm4, is well-conserved in the Cmr3 structure. The corresponding beta-hairpin of Cmr3 binds the D1 domain of Cmr2, as observed in the Cmr2-Cmr3 complex structure. Furthermore, it is suggested that the hairpin of Csm4 is responsible for the interaction with Csm1 (ortholog of Cmr2) [1].
