Imidazolepropionate, a nonmetabolizable inducer for the histidine-degrading enzymes in Aerobacter aerogenes.

An important contribution to the study of inducible enzymes has been the observation that nonmetabolizable compounds similar in structure to the inducer may also cause induction (l-3). When the inducer is metabolized, the cell not only is provided with new enzymes but also with the products of the enzymatic action and ultimately with a new source of carbon and energy. With the use of compounds which can serve as gratuitous inducers, the specific phenomenon of enzyme induction may be dissociated from these other effects. The nonmetabolizable ,&galactosides are the most familiar examples of such inducers. In Escherichia coli these compounds induce the enzyme P-galactosidase and a specific permease for galactoside transport (1,4-6). Because these galactosides accumulate unchanged in the cell they have been particularly valuable for the study of the specific permease (57). In Aerobacter aerogenes the enzyme L-histidine ammonia-lyase is induced by histidine, its substrate, as well as by urocanate, the product of the irreversible reaction catalyzed by this enzyme (8). Although urocanate is a grat,uitous inducer for the lyase it is further metabolized by the cell (9). The present work introduces the use of imidazole-4(5)-propionate as a nonmetabolizable inducer of the histidine-degrading enzymes. From the studies with this compound evidence was obtained for the existence of an inducible permease which appears to be specific for urocanate.