Isomaltose formed by α-glucosidases triggers amylase induction in Aspergillus nidulans

Among various alpha-glucobioses examined, isomaltose was the most effective inducer for amylase synthesis in Aspergillus nidulans. Amylase induction by maltose was completely inhibited by addition of castanospermine or cycloheximide, while induction by isomaltose was not affected by the inhibitors, suggesting that amylase induction by maltose requires inducible alpha-glucosidases. Disruption of the alpha-glucosidase A gene (agdA), the alpha-glucosidase B gene (agdB), or both genes did not abolish maltose-dependent induction, although amylase production induced by maltose decreased about 2-fold in the agdA/agdB double disruptant, compared with that in the agdB disruptant at all concentrations tested. Upon induction by isomaltose, amylase synthesis was enhanced considerably in the agdB and agdA/agdB disruptants. Even at 3 nM, isomaltose induced amylase production in the double disruptant, supporting the suggestion that isomaltose is a physiological inducer for amylase. Therefore, maltose must be converted to isomaltose by alpha-glucosidases prior to triggering amylase synthesis, but no specific alpha-glucosidase is required for amylase induction by maltose. Probably any alpha-glucosidases having isomaltose-forming activity, including AgdA and AgdB, may participate in amylase induction by maltose.