Characterization of genes in the sterigmatocystin gene cluster and their role in fitness of Aspergillus nidulans

Abstract

Sterigmatocystin (ST) is the penulitmate precursor of aflatoxin (AF) and a carcinogenic mycotoxin produced by A. nidulans, a fungus well established as a genetic model system. Previously, a 60 kb ST gene cluster in A. nidulans was identified and characterized but not all of the stc enzymatic genes have been identified in the ST biosynthetic pathway. Disruption of stcC resulted in no ST production and no visible 1-hydroxyversicolorone (HVN) was detected in extracts of the mutant. Disruption of stcG resulted in accumulation of 5-hydroxyaverantin (HAVN), averufanin (AVNN), and ST. These results suggest stcC and stcG are required for ST production but the []stcG strain is leaky. To determine if ST/AF has an effect on fungal fitness, five isogenic strains, differing only in four ST cluster genes, ([]aflR, []stcJ, []stcE, and []stcU), were grown on agar plates containing glucose minimal media or on live corn seeds. Relative fitness was determined as a difference in the number of conidia harvested after incubation for seven days at 37⁰C. In the media experiment and under the high concentration of inoculum on corn seeds, ST production was beneficial to fungal fitness. The incremental increase in fitness exhibited in the media experiment suggest that advancement along the pathway increases fitness rather than fitness differences being attributable simply to the impact of knockout mutations. Under the low concentration of inoculum on corn seeds, the ST producing and []stcJ strains showed the greatest fitness. These results suggest that ST (and presumably AF) production contributes to the fitness of Aspergillus under the environmental conditions tested.