Exogenous and Endogenous Induction of the Histidine-degrading Enzymes in Aerobacter aerogenes

Three different imidazole compounds are known to induce the enzymes that degrade histidine in Aerobackr aerogenes (l-4). Two of them, histidine and urocanate, are, respectively, the substrate and product of the first enzyme, histidine ammonialyase; the third, imidazolepropionate, is a nonmetabolizable analogue of urocanate. Previous studies with this system did not discriminate between the possibility that histidine and urocanate act as inducers independently, or that histidine is an inducer solely by virtue of its conversion to urocanate. In order to distinguish between these alternatives a number of mutants of A. aerogenes unable to degrade histidine were isolated. Some of the mutants are impaired in the ability to convert histidine to urocanate because of a low level of the histidine ammonia-lyase: in these mutants histidine is a poor inducer, whereas urocanate and imidazolepropionate behave as in the parent organism. Other mutants lack the enzyme urocanase; these mutants have high levels of the other histidine-degrading enzymes even in the absence of added inducer. These observations suggested that exogenously supplied or endogenously produced urocanate, rather than histidine, is the inducer. This view is strongly supported by a detailed biochemical study of these mutants.