The ribonuclease activity of Csm6 is required for anti-plasmid immunity by Type III-A CRISPR-Cas systems.

CRISPR-Cas systems provide prokaryotes with RNA-based adaptive immunity against viruses and plasmids. A unique feature of Type III CRISPR-Cas systems is that they selectively target transcriptionally-active invader DNA, and can cleave both the expressed RNA transcripts and source DNA. The Type III-A effector crRNP (CRISPR RNA-Cas protein complex), which contains Cas proteins Csm1-5, recognizes and degrades invader RNA and DNA in a crRNA-guided, manner. Interestingly, Type III-A systems also employ Csm6, an HEPN family ribonuclease that does not stably associate with the Type III-A effector crRNP, but nevertheless contributes to defense via mechanistic details that are still being determined. Here, we investigated the mechanism of action of Csm6 in Type III-A CRISPR-Cas systems from Lactococcus lactis, Staphylococcus epidermidis, and Streptococcus thermophilus expressed in Escherichia coli. We found that L. lactis and S. epidermidis Csm6 cleave RNA specifically after purines in vitro, similar to the activity reported for S. thermophilus Csm6. Moreover, L. lactis Csm6 functions as a divalent metal-independent, single strand-specific endoribonuclease that depends on the conserved HEPN domain. In vivo, we show that deletion of csm6 or expression of an RNase-defective form of Csm6 disrupts crRNA-dependent loss of plasmid DNA in all three systems expressed in E. coli. Mutations in the Csm1 palm domain, which are known to deactivate Csm6 ribonuclease activity, also prevent plasmid loss in the three systems. The results indicate that Csm6 ribonuclease activity rather than Csm1-mediated DNase activity effects anti-plasmid immunity by the three Type III-A systems investigated.