Inducers of aflatoxin biosynthesis from colonized maize kernels are generated by an amylase activity from Aspergillus flavus

ABSTRACT Aflatoxin biosynthesis was induced by compounds in filtrates (EF) obtained from cultures consisting of ground maize kernels colonized by Aspergillus flavus. The inducing activity increased to a maximum at 4 days of incubation and then decreased. Amylase activity was detected in the EF, suggesting that the inducers are products of starch degradation (glucose, maltose, and maltotriose). Analysis of the enzyme by isoelectric focusing electrophoresis indicated a single α-amylase with a pI of 4.3. No maltase or amyloglucosidase was detected in the EF. High-pressure liquid chromatography analysis of the EF indicated the presence of glucose, maltose, and maltotriose in near-equal molar concentrations (about 15 mM). With a β-glucuronidase (GUS) reporter assay consisting of A. flavus transformed with an aflatoxin gene promoter-GUS reporter gene fusion to monitor induction of aflatoxin biosynthesis, the minimum concentration of glucose, maltose, or maltotriose that induced measurable GUS activity was deter...