Fig. 6: Structural and mechanistic comparisons between type III-A and type III-E effector complexes.

Overall architecture of the type III-A (PDB 6MUR) (a) and III-E (c) effector complex and CRISPR-loci. b Model of type III-A interference during the immune defense. The type III-A muti-subunit effector complex recognizes the invasive non-self RNA targets with 3′-flanking sequence noncomplementary to the 5′-repeat tag, triggering the non-specific ssDNA cleavage and cOA synthesis by Cas10 protein. The produced cOA activates the Csm6 RNase activity, which is crucial for type III-A immunity. The Cas10 enzymic activities are switched off upon the subsequent target RNA cleavage by Csm3 subunits. Binding of the self RNA containing an 3′-anti-tag sequence complementary to the 5′-repeat tag of crRNA could not activate Cas10 activities, but still could be cleaved by Csm3 subunits. d Model of type III-E interference during anti-phages or -plasmids infection. The type III-E single-protein effector directly interacts with a caspase-like protease TPR-CHAT, which adopts an autoinhibitory state. Binding of non-self RNA target activates the protease activity of the CHAT domain, leading to cleavage of target Csx30 protein, which forms a stable complex with RpoE. The resulting two products Csx30-C and RpoE-Csx30-N might be involved in the following antiphage defense. Cleavage of target RNA by Csm3 domains would shut down the protease activity. The 3′-anti-tag sequence takes a different binding route, leaving the protease to remain in an autoinhibitory inactive state. The bound self RNA targets could still be cleaved by Csm3 domains.