This protein family is typified by YPO2462 of Yersinia pestis and it is a CRISPR-associated (Cas) family strictly associated with the Ypest subtype of CRISPR/Cas locus. It is designated Csy4, for CRISPR/Cas Subtype Ypest protein 4. crRNA biogenesis in Pseudomonas aeruginosa requires the endoribonuclease Csy4, which binds and cleaves the repetitive sequence of the CRISPR transcript PMID:22522703. The crystal structure of Csy4 in complex with its cognate RNA reveals an unexpected recognition mechanism whereby Csy4 makes sequence-specific interactions in the major groove of the CRISPR repeat stem-loop. Together with electrostatic contacts to the phosphate backbone, these enable Csy4 to selectively bind and cleave pre-crRNAs PMID:20829488.