The DMSO reductase of Rhodobacter capsulatus contains a pterin molybdenum cofactor (Moco) and is located in the periplasm. DNA sequence analysis identified four genes involved in the biosynthesis of the Moco (moaA, moaD, moeB and moaC) immediately downstream of the dor (DMSO respiratory) gene cluster. Rhodobacter capsulatus MoaA was expressed in Escherichia coli as a His(6)-tagged protein. Although, the expressed protein formed inclusion bodies, EPR spectroscopy showed that MoaA contains a [3Fe-4S] cluster. A moaA mutant was constructed and its phenotype indicates that the Moco biosynthetic gene cluster downstream of the dor operon is specific for the biogenesis of DMSO reductase. Two forms of DMSO reductase were purified by immunoaffinity chromatography from the moaA mutant. A mature form of DMSO reductase was located in the periplasm and a precursor form was found in the cytoplasm.