SummaryTOL plasmid PWWO of Pseudomonas putida contains two operons that specify a pathway for the degradation of aromatic hydrocarbons. The ‘upper’ operon encodes enzymes for the oxidation of toluene to benzoate and xylenes to toluates, whereas themeta‐cleavage operon specifies the further oxidation of benzoate and toluates. Transcription of the upper pathway operon (s positively regulated by the XylR protein, which is activated by toluene/xylenes and their alcohol catabolic products, in combination with the NtrA protein, a sigma factor. Expression of themeta‐operon is positively controlled by the XylS protein which is activated by meta‐pathway substrates, and is Independent of NtrA protein. Expression of themetapathway is also Induced by toluene/xylene‐activated XylR protein via a cascade regulatory system in which this protein in combination with NtrA protein stimulates transcription from thexylSgene promoter. Hyper‐production of XylS protein in turn provokes high level expression of the meta‐operon, which is independent of meta‐pathway substrates. The two promoters, which are activated by the XylR and NtrA proteins, the upper pathway promoter and thexylSgene promoter, exhibit three regions of homology centred at –12(5′–TTGCTÃG–3′), –24(5′ ‐TGGCPuT –3) and –45(5′‐TTAAATÃGPuPuGCGPuTc‐3′), with respect to their principal transcription initiation points. The possible physiological significance of activated XylR‐protein‐induced expression of the meta‐operon throu