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The linkage of genes governing synthesis of enzymes functional in the conversion of l(+)-mandelate to benzoate in Pseudomonas aeruginosa was studied. These genes were found to be linked in a fashion consistent with regulatory mechanisms known to govern synthesis of this inducible pathway. Surprisingly, linkage was also observed between the mandelate genes and those governing the synthesis of functionally related enzymes that participate in the catabolism of anthranilate and benzoate. These latter genes are grouped in at least three regulatory units. Some theoretical explanations for high intensity of linkage among functionally related genes whose expression is not induced by a common metabolite are discussed.

The pathway of mandelate metabolism in Pseudomonas aeruginosa is composed of the following steps: l(+)-mandelate --> benzoylformate --> benzaldehyde --> benzoate. These three steps are unique to mandelate oxidation; the benzoate formed is further metabolized via the beta-ketoadipate pathway. The first enzyme, l(+)-mandelate dehydrogenase, is induced by its substrate. The second and third enzymes, benzoylformate decarboxylase and benzaldehyde dehydrogenase, are both induced by benzoylformate. The same benzaldehyde dehydrogenase, or one very similar to it, is also induced by beta-ketoadipate, an intermediate in the subsequent metabolism of benzoate. This dehydrogenase may also be induced by adipate or a metabolite of adipate. These conclusions have been drawn from the physiological and genetic properties of wild-type P. aeruginosa strains and from the study of mutants lacking the second and third enzyme activities.