Type III CRISPR systems synthesise cyclic
oligoadenylate (
cOA) second messengers in response to
viral infection of bacteria and archaea, potentiating an immune response by binding and activating ancillary effector nucleases such as Csx1. As these effectors are not specific for invading
nucleic acids, a prolonged activation can result in cell dormancy or death. Some archaeal species encode a specialised ring nuclease
enzyme (Crn1) to degrade
cyclic tetra-adenylate (cA4) and deactivate the ancillary nucleases. Some archaeal viruses and bacteriophage encode a potent ring nuclease anti-CRISPR, AcrIII-1, to rapidly degrade cA4 and neutralise immunity. Homologues of this
enzyme (named Crn2) exist in type III CRISPR systems but are uncharacterised. Here we describe an unusual fusion between cA4-activated CRISPR
ribonuclease (Csx1) and a cA4-degrading ring nuclease (Crn2) from Marinitoga piezophila. The
protein has two binding sites that compete for the cA4
ligand, a canonical cA4-activated
ribonuclease activity in the Csx1 domain and a potent cA4 ring nuclease activity in the C-terminal Crn2 domain. The cA4 binding affinities and activities of the two constituent
enzymes in the fusion
protein may have evolved to ensure a robust but time-limited
cOA-activated
ribonuclease activity that is finely tuned to cA4 levels as a second messenger of
infection.