Dicing defence in bacteria

Abstract

A newly discovered variation in the process by which bacteria resist invasion by foreign nucleic acids provides an intriguing parallel between the defence mechanisms of the different domains of life. See Article p.602 CRISPR is a microbial RNA-based immune system protecting against viral and plasmid invasions. The CRISPR system is thought to rely on cleavage of a precursor RNA transcript by Cas endonucleases, but not all species with CRISPR-type immunity encode Cas proteins. A new study reveals an alternative pathway for CRISPR activation in the human pathogen Streptococcus pyogenes, in which a trans-encoded small RNA directs processing of precursor RNA into crRNAs through endogenous RNase III and the CRISPR-associated Csn1 protein.