UP-REGULATION OF CELLOBIASE SECRETION BY THERMAL STRESS IN THE FILAMENTOUS FUNGUS PENICILLIUM CHRYSOGENUM

Abstract

Cellobiase (E.C. 3.2.1.21) controls the rate limiting step of cellulose hydrolysis in the production of cellulosic ethanol chiefly from agricultural biomass. The effect of thermal stress on secretion of cellobiase by the filamentous fungus Penicillium chrysogenum was studied. Exposure of 40 hours old mycelial culture to 42˚C temperature reduced mycelial growth by 10% and resulted in constricted hyphal morphology. The intracellular elucidation of heat stress was also confirmed by upregulation of specific activity of the stress marker protein disulphide isomerase. Concomitantly, heat stress also resulted in increased specific activity of extracellular cellobiase by 50% from 0.5 U/ml to 1 U/ml which was also confirmed through zymography after Native PAGE. The cellobiase retained in intracellular fraction also had higher specific activity indicating that more cellobiase was synthesized under thermal stress. Trehalose is synthesized as a protective osmolyte under physiological stress. However, it is enzymatically degraded as soon as stress subsides. Specific activity of intracellular trehalase slowly increased post stress which indicated gradual dissipation of stress induced trehalose accumulation. The results collectively suggested that the fungus responds to heat stress by resorting to increased production and secretion of cellobiase to mobilize more nutrients from the extracellular environment. The findings provided valuable insights for industrial upregulation of synthesis and secretion of cellobiose.