Four genes, dorC, dorD, dorB and dorR of the DMSO respiratory gene cluster of Rhodobacter capsulatus have been identified and sequenced. dorC encodes a pentahaem c-type cytochrome of the NirT class and the derived DorC protein sequence shows highest similarity to TorC from the Escherichia coli trimethylamine-N-oxide (TMAO) respiratory system. Mutagenesis of dorC resulted in the loss of a 46 kDa haem-staining polypeptide from membranes of R. capsulatus. dorD encodes a protein with highest sequence similarity to TorD from the E. coli TMAO respiratory system. DMSO reductase polypeptide (DorA) could not be detected in cell-free extracts of a dorD mutant and it is suggested that DorD has a role in stabilizing the DorA apo-protein prior to insertion of the pterin molybdenum cofactor. dorB encodes a protein with highest sequence similarity to NapD of Paracoccus denitrificans. Mutagenesis of dorB reduced the activity of DMSO reductase and led to the accumulation of a larger form of the enzyme that is presumed to represent a cytoplasmic precursor polypeptide. It is suggested that DorB has a role in the biogenesis of DMSO reductase prior to its secretion into the periplasm. dorR is transcribed in the opposite direction to dorC. The derived amino acid sequence of DorR indicates that it is a response regulator and mutation of dorR shows that it is essential for expression of the dorCDA operon. Expression of a chromosomal dorA::lacZ fusion was also dependent on the transcriptional regulator Fnr. The intergenic region between dorR and dorC contains four putative binding sites for DorR but no binding site for Fnr was identified.