Overexpression of amyA and glaA substantially increases glucoamylase activity in Aspergillus niger

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.

Identification of Glucoamylase cDNA Sequence of Saccharomycopsis (Syn. Endomycopsis) bubodii 2066

Saccharomycopsis (Syn. Endomycopsis) bubodii 2066 is an isolate from bubod, a starter used in making rice wine in northern Philippines. We have shown that the yeast has amylolytic activity on raw sago starch. In our attempt to identify the putative raw starch-digesting amylase in S. bubodii, we determined the cDNA sequence of a glucoamylase gene. One primer pair that was designed based on a glucoamylase of Saccharomycopsis fibuligera HUT7212 (GLU1, NCBI Accession Number L25641.1) produced a sequence of 1234 base pairs. To obtain a wider coverage, a primer walking strategy was carried out using four primer pairs designed based on GLU1 gene. The generated sequence of 1535 base pairs shows 98.7 to 100% homology when aligned with glucoamylase genes from four strains of S. fibuligera suggesting that this glucoamylase is highly conserved between the Saccharomycopsis species. This work further reports a gene sequence of glucoamylase derived from Philippine-isolated yeast. The sequence is deposited in GenBank and assigned the accession number KP068007.1. The gene may be heterologously expressed in Saccharomyces cerevisiae for possible utilization in the direct conversion of raw sago starch to bioethanol.