Extended Figure 7. SAM lyse

<strong>CRISPR systems are widespread in the prokaryotic world, providing adaptive immunity against mobile genetic elements (MGE) 1,2. Type III CRISPR systems, with the signature gene </strong><em><strong>cas10</strong></em><strong>, use CRISPR RNA (crRNA) to detect non-self RNA, activating the enzymatic Cas10 subunit to defend the cell against MGE either directly, via the integral HD nuclease domain 3-5 or indirectly, via synthesis of cyclic oligonucleotide (cOA) second messengers to activate diverse ancillary effectors 6-9. A subset of type III CRISPR systems encode an uncharacterised CorA-family membrane protein and an associated NrN family phosphodiesterase predicted to function in antiviral defence. Here, we demonstrate that the CorA associated type III-B CRISPR system from </strong><em><strong>Bacteroides fragilis</strong></em><strong> (BfaCmr) provides immunity against MGE when expressed in </strong><em><strong>E. coli</strong></em><strong>. However, BfaCmr does not synthesise cOA species on activation, instead generating a previously undescribed signalling molecule, SAM-AMP (3’-adenylyl-AdoMet) by conjugating ATP to S-adenosyl methionine via a phosphodiester bond. Once synthesised, SAM-AMP binds to the CorA effector, presumably leading to cell death by disruption of the membrane integrity. SAM-AMP is degraded by CRISPR associated phosphodiesterases or a SAM lyase, providing an “off switch” analogous to cOA specific ring nucleases 10. SAM-AMP thus represents a new class of second messenger for antiviral signalling, which may function in different roles in diverse cellular contexts. </strong>