Phylogenetic tree of archaeal and bacterial TrpA sequences. The two subtrees cluster genomes, which encode at least one trpB1 gene (S1, s1, i1_o1, or i1_o2 species) or which possess only genes of type trpB2 (S2, s2, i2_o2 species). The clusters were named TrpA1 or TrpA2, respectively. For abbreviations of sequence names, see Results. For colour code, see legend of Figure 1. For the acronyms of species-types, see legend of Table 2.