The biosynthesis of methylamine dehydrogenase (MADH) from Paracoccus denitrificans requires four genes in addition to those that encode the two structural protein subunits. None of these gene products have been previously isolated. One of these, mauG, exhibits sequence similarity to diheme cytochrome c peroxidases and is required for the synthesis of the tryptophan tryptophylquinone (TTQ) prosthetic group of MADH. A system was developed for the homologous expression of MauG in P. denitrificans. Its signal sequence was correctly processed, and it was purified from the periplasmic cell fraction. The protein contains two covalent c-type hemes, as predicted from the deduced sequence. EPR spectroscopy reveals that the protein as isolated possesses about equal amounts of one high-spin heme with axial symmetry and one low-spin heme with rhombic symmetry. The low-spin heme contains a major and minor component suggesting a small degree of heme heterogeneity. The high-spin heme and the major low-spin heme component each exhibit resonances that are atypical of c-type hemes and dissimilar to those reported for diheme cytochrome c peroxidases. MauG exhibited only very weak peroxidase activity when assayed with either c-type cytochromes or o-dianisidine as an electron donor. Fully reduced MauG was shown to bind carbon monoxide and could be reoxidized by oxygen. The relevance of these unusual properties of MauG is discussed in the context of its role in TTQ biogenesis.