FACTORS INFLUENCING THE INDUCTION OF ENDO-XYLANASE BY THERMOASCUS-AURANTIACUS

Fourteen carbohydrates were screened for their ability to induce xylanase in the newly isolated thermophilic fungus, Thermoascus aurantiacus. Among the carbohydrates screened, beech wood xylan, birch wood glucuronoxylan and methyl beta-D-xylopyranoside (MXP) appeared to induce xylanase. Low levels of constitutive enzymes were produced on all the compounds even in the absence of an added carbon source to the basal medium. A long lag period for enzyme induction was observed with all the inducers. This lag period was markedly reduced when the mycelia were starved prior to the addition of the inducer. The most powerful non-metabolizable inducer, MXP, induced the highest xylanase activity in a concentration up to 2.5 mg ml-1. The optimum initial pH and culture temperature for induced synthesis of xylanase were around pH 4.0 and 50-degrees-C, respectively. For pronounced synthesis of the enzyme organic nitrogen, in addition to an appropriate inducer, was necessary. The surfactant, Tween-80, had enhanced the enzyme secretion into the culture fluid. AgNO3, CoNO3, HgCl2, KMnO4, MnCl2 and SDS showed moderate to strong inhibitory effects on both the synthesis and activity of xylanase, while CuSO4, EDTA, SnCl and ZnCl2 showed a strong inhibitory effect on xylanase activity only. The enzyme induction was repressed by high concentration of glucose and this could not be reversed by exogenous cyclic-adenosine 3',5'-monophosphate (cAMP). Mixture of inducer (MXP) and repressor (glucose) produced a delayed induction compared with solutions of inducers only. The repression, however, was completely restored after the depletion of glucose level from the medium to the non-repressive level (approx. 0.2 mg ml-1).