Three DNA regions carrying genes encoding putative homologs of xanthine dehydrogenases were identified inEscherichia coli, named xdhABC, xdhD, andyagTSRQ. Here, we describe the purification and characterization of gene products of the yagTSRQ operon, a molybdenum-containing iron– sulfur flavoprotein from E. coli, which is located in the periplasm.