Submitted to: Fungal Genetics and Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 15, 2002
Publication Date: N/A
Interpretive Summary: Aflatoxin contamination in cottonseed, corn, peanuts, and tree nuts is a worldwide problem. Solutions to this problem can only come when scientists understand under what conditions and how the fungi (Aspergillus species) make the toxin. Previous research identified most of the genes in Aspergillus flavus and Aspergillus parasiticus that are required for formation of aflatoxins. These species are the two main fungal species that contaminate commercial crops. In order to better understand how environmental factors turn on aflatoxin synthesis, the structure of aflR, the main regulatory gene, was compared among five different species of Aspergillus capable of aflatoxin production. Differences among the species were found in DNA sequences that are recognized by proteins that mediate the fungal response to nitrogen and pH. Also, some of the species have DNA sequences that are recognized by a protein that controls sporulation. Using this information, it was possible to show that the Aspergillus species that contaminate crops may have evolved from ancestral species that are predominantly associated with insects. Results show that aflatoxin production probably is advantageous to the fungus under certain conditions that may only be a holdover when the fungi inadvertently invade plants. If this is true, competitor fungi incapable of aflatoxin production, should readily be able to displace aflatoxin producing fungi.
Technical Abstract: Aflatoxin contamination is a world-wide agricultural problem. Aflatoxin production requires expression of the biosynthetic pathway regulatory gene, aflR, which encodes a Cys6Zn2-type DNA-binding protein. Homologs of aflR from Aspergillus nomius, bombycis, parasiticus, flavus, and pseudotamarii were compared in order to investigate the molecular basis for variation among aflatoxin-producing taxa in the regulation of aflatoxin production. Variability was found in putative promoter consensus elements and coding region motifs, including motifs involved in developmental regulation (AbaA, BrlA), regulation of nitrogen source utilization (AreA), and pH regulation (PacC), and in coding region PEST domains. Some of these elements may affect expression of aflJ, a gene divergently transcribed from aflR, that also is required for aflatoxin metabolite accumulation. Significant incongruence (P<0.001) between the phylogenetic signals for the promoter and coding regions detected with the partition homogeneity test and the observed divergence in regulatory features among the taxa support the conclusion that aflatoxin production has evolved to respond to a variety of environmental stimuli under differential selective pressures. Phylogenetic analyses suggest that strains currently classified as A. flavus SBG are distinct species from A. flavus.