INDUCTIVE AND REPRESSIVE EFFECTS OF CARBON AND NITROGEN ON L-HISTIDINE AMMONIA-LYASE ACTIVITY IN A BLACK CHERNOZEMIC SOIL

The induction and repression of L-histidine NH3-lyase (EC 4.3.1.3) was examined in samples of a black chernozemic soil (typic Cryoboroll). Ammonium production in response to addition of excess histidine was used to estimate potential soil histidase activity, a measure of histidase content. Kinetic analysis of the decay of soil histidase following toluene addition was used to quantify components of histidase. Soil histidase was sensitive to substrate-specific enzyme synthesis as indicated by an increase in response to soil treatment with 100-mu-g g-1 of either histidine-C or urocanate-C. Increases of similar magnitude were not observed as a result of glucose and ammonium addition. We conclude that substrate-specific histidase synthesis was due to putative induction rather than histidine-specific growth, The increase in histidase following urocanate addition supported the tentative identification of measured activity as that of L-histidine NH3-lyase, the first enzyme of the hut (histidine utilization) operon. Complete repression of soil histidase synthesis requires 4000-mu-g g-1 of glucose-C. From the amounts of urocanate-C or glucose required for putative induction or repression, respectively we inferred that these mechanisms are masked in horizon level expression of soil histidase activity. Putative induction of histidase is likely to be restricted to microsites of substrate accumulation. Background soil histidase activity persisted in the presence of repressive concentrations of glucose. This activity was attributed to constitutive enzymes which we concluded were characteristic of histidase content in this soil under non-amended conditions.