A novel immunity system for bacterial nucleic acid degrading toxins and its recruitment in various eukaryotic and DNA viral systems

Zhang, Dapeng; Iyer, Lakshminarayan M.; Aravind, L.

Phyletic and phylogenetic analysis of the SUKH superfamily indicates three salient features, namely rampant lateral transfer between different branches of the bacterial tree, gene loss and lineage-specific expansion followed by divergence of the lineage-specific paralogs (Supplementary Data). This suggests that there is a notable trend for maintaining diversity within the SUKH superfamily that probably arises from selection for recognition of a diverse range of nucleic acid-modifying toxins. Although there are multiple distinct types of immunity proteins known from plasmid-borne bacteriocins and CDI systems, most show very limited phyletic patterns. For example the CdiI toxin seen in several CDI systems is entirely limited to proteobacteria (25) . We observed that it is a protein with two TM segments that is likely to form a membrane channel (Supplementary Data) and have a mode of action very distinct from the SUKH superfamily. As only the SUKH superfamily and, to certain extent, the SuFu superfamily show a pattern of wide dissemination across bacteria it is likely that only these scaffolds can support sufficient diversification that goes hand in hand with the polymorphism of the toxin domains.