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1. The ability to oxidize quinate is elicited in Acinetobacter calco-aceticus not only by growth on quinate but also by growth on protocatechuate or its precursors such as shikimate and p-hydroxybenzoate. Accordingly the synthesis of a quinate dependent dehydrogenase was found to be induced by protocatechuate. Moreover, this enzyme seems to belong to the same coordinate block of enzymes responsible for the conversion of shikimate to β-ketoadipyl-CoA. 2. Analogies between quinate and shikimate oxidation m A. calco-aceticus have been substantiated by showing that both quinate dehydrogenase and shikimate dehydrogenase activities rely on a single enzyme. 3. Several auxotrophs that have lost the ability to utile both quinate and shikimate as sole source of carbon and energy have been isolated. Particutarly relevant for this study are two of these mutants in which the dehydrogenase is severely impaired and correspondingly their ability to oxidize the mentioned hydroaromatic compounds is substantially reduced. The properties of these auxotrophs and their revertants confirm the key role of the dehydrogenase in the dissimilation of both quinate and shikimate. The rationale of the control of this enzyme by protocatechuate is discussed. [ABSTRACT FROM AUTHOR]