Rhodococcus erythropolis TA421 has seven genes encoding 2,3-dihydroxybiphenyl dioxygenase. Three of them are located on a linear plasmid that is necessary for growth on biphenyl. Sequence analysis revealed that two of the plasmid-derived genes (bphC3 and bphC4) are accompanied by the genes for other enzymes involved in the biphenyl degradation pathway, i.e. biphenyl dioxygenase, dihydrodiol dehydrogenase, and hydrolase.