In bioenergetic research Paracoccus denitrificans has been used as an interesting model to elucidate the mechanisms of bacterial energy transduction. Genes for protein complexes of the respiratory chain and for proteins which are involved in periplasmic electron transport have been cloned and sequenced. Conjugational gene transfer has allowed the construction of site-specific mutant strains. Complementation experiments did not only open the field for site-directed mutagenesis and investigation of the structure/function relationship of the various electron-transport proteins, but also allowed first insights into processes like oxygen-dependent gene regulation or the assembly of electron-transport complexes. Also data will be presented that characterize two restriction-/modification systems, the codon usage and the promoter sequences of Paracoccus. Details will be given about the extrachromosomal localization of a duplicated cytochrome oxidase subunit I gene on one of the Paracoccus megaplasmids.