Gene organization of the first catabolic operon of TOL plasmid pWW53: production of indigo by the xylA gene product.

The entire operon coding for the enzymes responsible for conversion of toluenes to benzoates has been cloned from TOL plasmid pWW53 and the position of the genes accurately located. The coding region was 7.4 kilobase pairs (kbp) long, and the gene order was operator-promoter region (OP1)-a small open reading frame-xylC (1.6 kbp)-xylA (2.9 kbp)-xylB (1.8 kbp). Within the coding region there was considerable homology with the isofunctional region of the archetypal TOL plasmid pWW0. A central region of 2.9 kbp complemented an xylA (for xylene oxygenase) mutant of Pseudomonas putida mt-2 and was also capable of conferring the ability to convert indole to indigo on strains of Escherichia coli and P. putida. This reaction has been reported previously only for dioxygenases involved in aromatic catabolism but not for monooxygenases. It is proposed that the region encodes xylene oxygenase activity capable of direct monohydroxylation of indole to 3-hydroxyindole (oxindole), which then spontaneously dimerizes to form indigo.