CRISPR-Cas9 cleaves DNA, enabling genome editing. Surprisingly, three studies have concurrently uncovered an intrinsic RNA cutting activity of Cas9. Cas9 cuts endogenous RNA in some bacteria and, when programed, targets RNA in test tubes as well. Strutt et al. showed that Cas9 expression in a heterologous bacterium protects against infection by an RNA virus. Rousseau et al. showed that the activity can be switched off by virus-derived anti-Cas9 proteins. Dugar et al. profiled the cellular RNAs that are subjected to endogenous Cas9 targeting. Although how it is used to fend off intruders or regulate gene expression by bacteria remains to be investigated, this newly identified activity of Cas9 provides an exciting tool in our increasing arsenal for modulating RNA metabolism.
