Distribution of the sequences of characterized (experimental/predicted) TonB-dependent transporters (TBDTs). The grey dashed frame from Figure 1B is shown here containing expTBDTs and pTBDTs, which are marked by colored symbols and a number, which corresponds to the numbering in column 1 (see Additional file 1, [14–19, 22, 34–124]). Symbols are used to enhance the readability of the figures and are explained on the bottom right. The dashed frames are shown in a magnified view on the bottom. Circles define regions without functionally characterized TBDTs. For regions XI and XII the substrates are indicated on the left. The region numbering is explained in the text.