CRISPRs extending their reach: prokaryotic RNAi protein Cas9 recruited for gene regulation.

A Cas protein from the CRISPR defence system against foreign DNA, also functions in endogenous gene regulation. Sampson et al (2013) have revealed that in pathogenic Francisella bacteria, the Cas9 protein guided by small RNAs represses the mRNA of a lipoprotein. This novel mechanism of post-transcriptional regulation enables the infecting bacteria to evade the TLR2-based innate immune response of its host. Thus, reminiscent of eukaryotic RNAi where some proteins facilitate both genome defence and gene regulation, a central prokaryotic RNAi protein not only destroys invading DNA but also controls mRNA expression.