Induction, production, repression, and de-repression of exoglucanase synthesis in Aspergillus niger

Abstract

The influence of carbon and nitrogen sources on the production of cellulases was investigated. The enzyme production was variable according to the carbon source. Levels of β-cellobiohydrolase (CBH) were minimal in the presence of even low concentrations of glucose. Enzyme production was stimulated by other carbohydrates. The enzyme is subject to carbon source control by easily metabolizable sugars. Wheat bran and cellulose were the most effective promoters of β-cellobiohydrolase and filter paperase (FPase) activities respectively, followed by rice bran. Exogenously supplied glucose inhibited the synthesis of the enzyme in cultures of A. niger growing on wheat bran. In defined medium with cellobiose, the cellobiohydrolase titres were 2- to 110-fold higher with cells growing on monomeric sugars and 1.5 times higher than cells growing on other disaccharides. It appeared that synthesis of β-cellobiohydrolase varied under an induction mechanism, and a repression mechanism which changed the rate of synthesis of β-cellobiohydrolase and FPase in induced over non-induced cultures. In this organism, substantial synthesis of β-cellobiohydrolase can be induced by cellobiose, cellodextrin, cellulose or cellulose and hemi-cellulose containing substrates which showed low volumetric substrate uptake rate. The organism required limiting concentration of carbon, nitrogen or phosphorous for production of β-cellobiohydrolase and FPase. During growth of A. niger on wheat bran, maximum volumetric productivities ( Q p) of β-cellobiohydrolase and FPase were 39.6 and 32.5 IU/l h and were significantly higher than the values reported for some other potent fungi and bacteria. The addition of actinomycin D (a repressor of transcription) and cycloheximide, (a repressor of translation) completely repressed CBH/FPase biosynthesis, suggested that the regulation of CBH synthesis in this organism occurs at both transcriptional and translational level. Thermodynamic studies revealed that the culture exerted protection against thermal inactivation when exposed to different fermentation temperatures.