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Abstract
The CRISPR-Cas sytem provides many prokaryotes with acquired resistance to genome invaders such as viruses and other mobile genetic elements. This system uses small RNAs derived from Clustered Regularly Interspaced Short Palindromic Repeat (CRISPR) loci encoded in the host genome as well as Cas (CRISPR-associated) proteins. We have identified an endoribonuclease, Cas6, that plays a role in CRISPR (cr)RNA biogenesis. Cas6 interacts with sequence elements in the 5 region of precursor crRNAs and cleaves within the 3 region of the repeat. The cleavage was found to be divalent metal ion independent and occurs on the 5 side of the phosphodiester bond. Furthermore, the active site of Cas6 appears to consist of a triad of amino acids composed of conserved tyrosine, histidine, and lysine residues.