Three multicomponent oxygenases involved in the degradation of ptoluenesulfonate and p-toluenecarboxylate and the regulation o f their synthesis have been examined in three strains (T-2, PSB-4 and TER-1) of Cornamonas testosteroni. Strain T-2 utilizes p-toluenesulfonate as a source of carbon and energy for growth via p-sulfobenzoate and protocatechuate, and ptoluenecarboxylate via terephthalate and protocatechuate, and ha5 the unusual property of requiring the reductase (TsaE) of the toluenesulfonate methyl monooxygenase system (TsaMB) in an incompletely expressed sulfobenzoate dioxygenase system (PsbAC) [Schlafli Oppenberg, H. R. , Chen, G., terephthalate via protoeateehuate. Mutant TER-1, derived from strain T-2, utilized only terephthalate via protocatechuate. We detected no enzymes of the pathway from toluenesulfonate to sulfobenzoate in strains PSB-4 and TER-I , and Confirmed by PCR and Southern blot analysis that the genes (tsaMB) encoding toluenesulfonate monooxygenase were absent. We concluded that, in strain PSB-4, the regulatory unit encoding the genes for the conversion of toluenesulfonate to sulfobenzoate was missing. and that generation of mutant TER-1 involved deletion of this regulatory unit and of the regulatory unit encoding desulfonation of sulfobenzoate. The degradation of sulfobenzoate in strain PSB-4 was catalysed by a fully inducible sulfobenzoate dioxygenase system ( PsbAC, , , , ) , which, after purification of the oxygenase component (PsbA,,), turned out to be indistinguishable from the corresponding component from strain T-2 (PsbA, , ).Reductase P s b C , , , . which w e could separate but not purify, was active with oxygenase PsbA, , , , and Psbh-,. Oxygenase P s b A , , , was shown by electron paramagnetic resonance spectroscopy to contain a Rieske [2Fe-2S] centre. The enzyme system oxygenating terephthalate was examined and the oxygenasc component purified and characterized. The oxygenase component in strains T-2 (and mutant TER-I) and PSB-4 were indistinguishable. The reductase component, which w e separated but failed to purify, was active with the oxygenase from all strains. Gains and losses of blocks of genes in evolution is discussed.