1. The transfer from benzenesulphonate to benzoate as a growth substrate for Pseudomonas aeruginosa strain A resulted in a change in the enzymic route by which catechol was degraded; at intermediate stages it was possible to obtain cells containing the enzymes of both the ‘ortho’ and ‘meta’ metabolic pathways. 2. A similar result was effected by the reverse transfer, benzoate to benzenesulphonate. 3. Catechol itself always elicited a catechol 2,3-oxygenase in uninduced cells, but the product of this reaction, 2-hydroxymuconic semialdehyde, and biochemically related compounds such as 4-hydroxy-2-oxovalerate, unexpectedly induced a catechol 1,2-oxygenase. 4. Both types of catechol oxygenase are strongly repressed by the metabolic end products of both the ‘ortho’ and ‘meta’ pathways, but there was no inhibition of enzymic activity by these end products.

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