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A wild-type naphthalene-degrading strain Pseudomonas putida RKJ1 and two recombinant strains each of Ps. putida and Escherichia coli carrying the genes for naphthalene degradation on a recombinant plasmid pRKJ3, produced indigo and indirubin pigments from indole. Naphthalene, salicylate and IPTG induced cells of naphthalene-degrading recombinant bacteria produced up to two times higher indigo compared with the uninduced cells. The maximum rates of indigo formation by Ps. putida RKJ1, Ps. putida RKJ5/pRKJ3, Ps. putida KT2442/pRKJ3, E. coli TB1/pRKJ3 and E. coli AB1157/pRKJ3 were 0.60, 0.80, 0.60, 1.20 and 1.50 nmol min−1 mg dry biomass−1, respectively, using indole as the substrate. The apparent Km values of indigo formation by these same bacteria were 0.22, 0.15, 0.10, 0.21 and 0.20 mmol l−1, respectively, again using indole as the substrate. The present study revealed that E. coli AB1157 was the most efficient of the hosts tested for the expression of the plasmid encoded genes (pRKJ3) from the wild-type strain Ps. putida RKJ1. In addition, both recombinant E. coli strains were capable of producing indigo directly from nutrient medium.