Fig. 4.—

Plasmid pLbaR of R. felis str. LSU-Lb encodes an RTX-like T1SS. (A) Schema comparing the structure of the RTX T1SS and associated genes from plasmid VIBNI_pA of Vibrio nigripulchritudo (NC_015156) with T1SSs and associated genes from plasmid pLbaR of R. felis str. LSU-Lb, and also the chromosome and plasmid pcBtQ1 of Cardinium endosymbiont cBtQ1 of Bemisia tabaci (CBQZ010000007). Tan shading indicates high similarity (%ID) across T1SSs. Blue shading illustrates similarity (%ID) across Rickettsia and Cardinium only. Gene colors as follows: Red, T1SS ABC transporters; blue, T1SS membrane fusion proteins (MFP); green outer membrane efflux protein (OEP); gray, RTX-specific proteins; white, hypothetical protein; yellow, PD-(D/E)XK nuclease family transposase (pfam12784); black, toxins. A dashed box illustrates the close association of the MARTX (V. nigripulchritudo) and RHS-like (R. felis) toxins with the T1SS; the RHS-like toxins of Cardinium are not linked to the T1SS, but share similarity with the RHS-like toxin of R. felis (supplementary fig. S9, Supplementary Material online). (B) Model for the canonical T1SS illustrating the tripartite structure formed by the trimeric OEP (green), IM/periplasmic MFP (blue), and dimeric IM ABC transporter (red). Bridging the asymmetry between the OEP and ABC transporter oligomers, the MFP is shown as a putative hexamer with the N-terminal sequences in the cytoplasm (Lee et al. 2012). All sequenced Rickettsiales genomes encode an Apr-like T1SS (Gillespie 2014); the distinct RTX-like T1SS of R. felis str. LSU-Lb is predicted to utilize a second ABC transporter, characteristic of atypical RTX-T1SSs (Boardman and Satchell 2004; Lee et al. 2008; Li et al. 2008). (C) Phylogeny estimation of T1SS ABC transporters. Three clades of ABC transporters correspond to a recent classification (Kanonenberg et al. 2013). The gray box illustrates the Group 3 ABC transporter (AprD-like) of the T1SS encoded within all Rickettsiales genomes (Gillespie 2014). Within the Group 2 transporter clade, the duplicate ABC transporters of RTX and RTX-like T1SSs are within red boxes, with the sequences encoded on plasmid pLbaR highlighted yellow. See text for alignment and tree-building methods. Tree is final optimization likelihood: (−59,413.119327) using WAG substitution model with GAMMA and proportion of invariant sites estimated. Brach support is from 1,000 bootstrap pseudoreplications.