Schematic phylogenetic (unrooted neighbor joining) tree of the molybdenum subunits of the DMSO reductase superfamily. Arsenite oxidase (Aro) forms a clade distant from arsenate reductases (Arr) with the dichotomy Bacteria (in dark grey)/Archaea (in light grey) indicating that the origin of this enzyme pre-dates the divergence of domains. The same dichotomy is also observed within the Psr/Phs and the Ttr clades. A corresponding dichotomy is less obvious for the Unk clade and clearly absent in the Arr family. The operons of all four families: Psr/Phs, Ttr, Unk and Arr, have three genes in common denoted A, B and C, coding for the molybdo-(with the TAT signal marked in black), iron-sulfur- and anchor-subunits, respectively. These three genes are arranged in the Psr/Phs operon in the same order as in the Escherichia coli dms operon, i.e. ABC. In the case of the arr operon the order is CAB, C occasionnally missing (marked by #). In the case of the ttr and unk (when C is present) operon, the order is BCA. In this family a subgroup (highlighted by the hatched circle), lacks the C subunit and the TAT signal on the A subunit (marked by*) and contains abnormally large subunit B and therefore possibly represents a separate type of enzyme.