Improvement of cellulosic biomass-degrading enzyme production by reducing extracellular protease production in Aspergillus aculeatus

Abstract

We investigated the effects of deleting major extracellular protease-encoding genes on cellulolytic and xylanolytic enzyme production in Aspergillus aculeatus. We first investigated the effect of prtT deletion, a positive transcription factor for extracellular protease-encoding genes in Aspergillus, on extracellular protease production in A. aculeatus. Genetic analysis indicated that among the major extracellular proteases, pepIIa and pepIIb are controlled by PrtT, but pepI is not. Thus, we generated a mutant with deletion of the two genes prtT and pepI (ΔprtTΔpepI) and one with deletion of the three genes pepI, pepIIa, and pepIIb (ΔpepIΔIIaΔIIb). Extracellular protease activities decreased in both ΔprtTΔpepI and ΔpepIΔIIaΔIIb to 3% of that in the control strain (MR12). Comparative time-course analyses indicated that endoglucanase activity in ΔprtTΔpepI increased to double that in MR12. Xylanase activities increased in both ΔprtTΔpepI and ΔpepIΔIIaΔIIb to fourfold higher than that in MR12 at maximum. β-Glucosidase activities were increased in ΔprtTΔpepI and ΔpepIΔIIaΔIIb 1.3- and 1.4-fold higher than that in MR12 at maximum, respectively. Residual activities of endoglucanase, xylanase, and β-glucosidase after 7 days of incubation at 37°C in the culture supernatant were 63%, 36%, and 48% of the original in MR12. Residual endoglucanase activities were more than 80% of the original in ΔprtT, ΔprtTΔpepI, and ΔpepIΔIIaΔIIb. Residual xylanase activities were not improved in all test strains. β-Glucosidase remained almost 97% of the original in ΔprtTΔpepI. These findings indicated that the reduction of extracellular proteases effectively improved cellulolytic and xylanolytic enzyme production and stability in A. aculeatus.