The purpose of this study was to obtain an engineered Aspergillus niger strain with high glucoamylase activity by overexpressing the glucoamylase gene glaA and α-amylase gene amyA in A. niger CICC2462. Three recombinant strains containing a single copy of amyA (1A), containing two copies of amyA (2A), and coexpressing amyA and glaA (AG), respectively, were constructed. The transcript levels of amyA in 1A and 2A were increased by 2.95 folds and 3.09 folds, respectively. The levels of amyA and glaA in AG were increased by 1.21 folds and 2.86 folds, but the maximum extracellular glucoamylase activities did not differ significantly. In addition, after 1% casein phosphopeptides (CPPs) was added to the fermentation medium, the maximum extracellular glucoamylase activities for strains 1A, 2A, and AG were 35,200, 37,300, and 40,710 U/ml, respectively, which were significantly higher than that of the parental strain CICC2462 (28,250 U/ml), while CPPs alone had no effect on the parental strain CICC2462. We demonstrate that overexpression of amyA and glaA substantially increases the expression and secretion of glucoamylase in A. niger, and CPPs effectively improves the yield of glucoamylase in recombinant A. niger strains overexpressing amyA and glaA. The newly developed strains and culture methods may have extensive industrial applications.
Keywords: Aspergillus niger; α-amylase; casein phosphopeptides; glucoamylase; secretory expression.
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