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 possessing CRISPR-type immunity encode Cas proteins. This study now describes an alternative pathway in Streptococcus pyogenes that employs trans-encoded small RNA that directs the processing of precursor RNA into crRNAs through endogenous RNase III and the CRISPR-associated Csn1 protein.
- Elitza Deltcheva
- Krzysztof Chylinski
- Emmanuelle Charpentier