NJ-phylogram of the molybdenum subunits of the DMSO reductase family. Almost all the represented enzymes harbor the TAT signal of export to the periplasm. Enzymes lacking this signal are marked by "*". This is the case of entire families (Nas, Nar, Aro or FdhH) or subfamilies (Unk). Arsenite oxidase (Aro) forms a separate and distant clade and is therefore used as outgroup. Arsenate reductase (Arr), the new uncharacterized molybdoenzyme (Unk, see the text), tetrathionate reductase (Ttr) and polysulfide reductase/thiosulfate reductase (Psr/Phs) form distinct but related clades. Whereas representatives of the Psr/Phs clade all contain the anchor subunit, some representatives of the Unk and Arr clades (marked with #) were lacking this subunit. aOnly the sequence of the A subunit of Bacillus arsenicoselenatis is known. bThe Arr of Sulfurospirillum barnesii has been demonstrated biochemically to be composed of three subunits but only the A subunit has been sequenced. cThe Arr of Alkaliphilus oremlandii (formely Clostridium sp.) OhILAs and Alkaliphilus metalliredigenes have been published as composed of three subunits. We failed, however, to detect a typical ArrC subunit in the Arr operon. On the other hand our study revealed Arr operons in Geobacter lovleyi SZ, Shewanella sp. W3-18-1 and Alkalilimicola ehrlichei MLHE-1, the two first without and the last with the third subunit C, respectively.