Phylogenetic tree of SMR protein family members selected from Archaea and Bacteria. The unrooted phylogenetic tree is based on a NJ analysis of 338 SMR protein sequences identified from various Archaeal and Bacterial species. Due to the large number of sequences and tight clustering of branches within this tree, genus and species names were omitted and taxa were described according to their bacterial class. NCBI accession numbers of all SMR protein sequences and their genus and species names are shown in Additional Files 1 and 3. Plasmid and integron encoded SMR proteins are underlined and E. coli (Eco-EmrE, Eco-SugE, and Eco-YdgE/Eco-YdgF) and B. subtilis (Bsu-EbrA/Bsu-EbrB, Bsu-YkkC/Bsu-YkkD, Bsu-YvdR/Bsu-YvdS, and Bsu-YvaE/Bsu-YvaD) SMR homologues are indicated at their respective branch. The Archaeal Archaeoglobus Afu-QacE sequence served as an outgroup for this analysis. Bootstrap values were calculated but not shown on this tree; refer to Additional File 1 for confidence values at the respective nodes. Branches are coloured corresponding to their SMR subclass designation, PSMR (blue), SUG (green), and SMP (black) and individual PSMR members are highlighted according the following colours; YvaE (yellow), YvaD (Grey), YkkC/YvdS (violet), YkkD/YvdR (pink), YdgE (red), YdgF (orange), EbrA/EbrB (light blue).