Three-dimensional crystal structure of Acaligenes faecalis (Ellis et al., 2001) with residues identified by ET analyses mapped onto the protein. (a) Arsenite oxidase cluster showing the larger subunit I (blue), the smaller Rieske subunit (green), the [3Fe–4S] cluster (yellow), and the molybdenum-bis (pyranopterin guanine dinucleotide; Mo-bisPGD) cofactor (purple). (b–d) Arsenite oxidase large subunit illustrating the portion of the protein represented by our sequences (black) relative to the rest of the protein (gray), as well as the Mo-bisPGD (purple) cofactor and the [3Fe–4S] cluster (yellow). (b) Residues (light blue) conserved across all of the AroA sequences in our alignment. (c) Residues (green) divergent between the Aquificales group and the remaining sequences. (d) Residues unique to the Alphaproteobacteria (orange near the [3Fe–4S] cluster), divergent between the Proteobacteria and the Chloroflexi+ Aquificales group (orange near the Mo-bisPGD), or divergent among all groups (red).