Subgroups of Tae4 family proteins and their associated immunity proteins. Neighbour-joining tree calculated from a multiple sequence alignment of Ssp1, Ssp2 and other Tae4 homologues from different bacterial species (a full alignment is shown in Supplementary Fig. S6). For each Tae4 protein, the adjacently encoded candidate immunity protein was identified by genomic analysis and the S. marcescens Rap protein to which it was most closely related was determined. Tae4 homologues with associated immunity proteins similar to Rap1a (Tai4a) are shown by red circles, whereas those with immunity proteins of the Rap1b/Rap2a/Rap2b (Tai4) type are shown by squares (in light blue, green or dark blue for most similarity to Rap1, Rap2a or Rap2b, respectively). Apart from Ssp1 and Ssp2 from S. marcescens, the Tae4 homologues are labelled by organism and their identities are as follows (UniProt or genomic identifiers): Acinetobacter baumannii, B0VVE3_ACIBS; Agrobacterium tumefaciens, Atu4347; Burkholderia cenocepacia, Bcen_4030; Cronobacter sakazakii, ESA_03935; Enterobacter cloacae, ECL_01542; Enterobacter hormaechei, F5RYK9_9ENTR; Erwinia amylovora, EAMY_3018; Erwinia tasmaniensis, ETA_06210; Escherichia coli, ECEG_03250; Pantoea sp., S7A_11480; Pseudomonas syringae, Psyr_4040; Salmonella enterica serovar Newport, SNSL254_A0303; S. enterica serovar Typhimurium, STM0277; S. enterica serovar Typhi, STY0307; Serratia odorifera, D4E4R6_SEROD. Asterisks indicate the Tae4–Tai4 homologues for which structures have previously been reported (Zhang et al., 2013) and the open square indicates that this candidate immunity protein showed only weak similarity to Rap2a. Details of the Tae4 homologues and associated immunity proteins are given in Supplementary Table S1.