Aflatoxin formation and gene expression in response to carbon source media shift in Aspergillus parasiticus

Aflatoxins are toxic and carcinogenic polyketide metabolites produced by fungal species, including Aspergillus flavus and A. parasiticus. The biosynthesis of aflatoxins is modulated by many environmental factors, including the availability of a carbon source. The gene expression profile of A. parasiticus was evaluated during a shift from a medium with low concentration of simple sugars, Yeast Extract (YE), to a similar medium with sucrose, Yeast Extract Sucrose (YES). Total RNA and aflatoxins (B1, B2, G1, and G2) were quantified from fungal mycelia harvested pre- and post-shifting. When compared to YE media, YES caused temporary reduction of the aflatoxin levels detected at 3 hours post-shifting and they remained low well past 12 hours post-shift. Aflatoxin levels did not exceed the levels in YE until 24 hrs post shift, at which time point a 10-fold increase was observed over YE. Microarray analysis comparing the RNA samples from the 48 hr YE culture to the YES samples identified a total of 2120 genes that were expressed across all experiments, including most of the aflatoxin biosynthesis genes. One-way Analysis of Variance (ANOVA) identified 56 genes that were expressed with significant variation across all time points. Three genes responsible for converting norsolorinic acid to averantin were identified among these significantly expressed genes. The potential role of these genes involvement in the regulation of aflatoxin biosynthesis is discussed. Keywords: time course, media shift, Aspergillus, aflatoxin Each experiment consisted of one 48 hour YE sample coupled to either a 3, 6, 12, 24, or 48 hour sample from the YE to YES shift. The 48 hour YE to 3, 6, 12 and 24 hour YES comparisons were repeated with duplicate biological replicates and with duplicate dye-flip; the 0 hour to 48 hour comparisons were done in triplicate each with a dye-flip.