Organization of tryptophan-pathway genes in those archaea and bacteriathat possess trpEb_2. The 16S rRNA tree of the organisms that possess trpEb_2 is shown at the left. The tree was obtained from the SUBTREE program of the Ribosomal Database [41,42]. Each of the eight genes is color-coded differently. For economy of space trpD is D, trpC is C, etc. aroA in F. acidarmanus belongs to the AroAIβ grouping . Flanking genes are shown as boxes touching one another with intergenic spacing indicated. Minus values indicate translational coupling. The open gray box is a conserved hypothetical protein. Pointed box ends indicate the direction of transcription. Space between boxes indicates a lack of genetic linkage.