The Adaptive Degradation of L-Histidine by Paracolobactrum aerogenoides

SUMMARY: The characteristics of an induced, enzyme system responsible for the degradation of l-histidine by a soil organism were studied. Induction could be achieved by exposing cells to l-histidine, d-histidine, urocanate, β-alanyl-l-histidine or l-histidyl- l-histidine, but not by exposure to a variety of other imidazoles. The kinetics of induction by l- and d-histidine differed considerably and the d-isomer was shown to be metabolized only very slowly by fully induced cells. Chloramphenicol and dl-p-fluorophenylalanine strongly inhibited enzyme synthesis, whilst a variety of purine and pyrimidine analogues were without effect. Nitrogen starvation of noninduced cells decreased but did not completely prevent the appearance of the enzyme system on induction with d-histidine. Non-induced cells possess a low level of ‘basal’ enzymes before exposure to an inducing substance.