The degradation of aromatic compounds by bacteria is dependent upon specific catabolic operons. The unique car locus isolated from Sphingomonas CB3 encodes the first four enzymes involved in the catabolism of the azaarene carbazole. These include a class II three-component dioxygenase enzyme system, a dihydrodiol dehydrogenase, an extradiol (meta-cleavage) dioxygenase, and a hydrolase. Homology of deduced amino acid sequences is closer to corresponding biphenyl catabolic genes than to previously characterised carbazole degradation genes. Gene arrangement is also identical to that found in some bph loci. The car genes are transcribed when carbazole is utilised as a sole carbon source, and although biphenyl does not serve as a growth substrate for Sphingomonas CB3 it is able to act as a non-metabolisable inducer of the car locus. Ecologically the car genes were detected in polycyclic aromatic hydrocarbon (PAH) contaminated soil associated with a former town gas site.
