Submitted to: Mycological Research
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/8/2003
Publication Date: 7/1/2003
Citation: KALE, S.P., CARY, J.W., BAKER, C., WALKER, D., BHATNAGAR, D., BENNETT, J.W. GENETIC ANALYSIS OF MORPHOLOGICAL VARIANTS OF ASPERGILLUS PARASITICUS DEFICIENT IN SECONDARY METABOLITE PRODUCTION. MYCOLOGICAL RESEARCH. 2003. V. 107. P. 831-840. Interpretive Summary: Aflatoxins are cancer causing compounds produced mainly by the molds called Aspergillus flavus and A. parasiticus. When these molds invade crops such as corn, cotton, peanuts, and treenuts prior to harvest and produce aflatoxins, they render the crops unsalable, causing millions of dollars worth of bosses to U.S. agriculture. Therefore, it is very important to find ways to control the production of these toxins and the growth of fungus in these crops. In this study we have found that there is a correlation between fungal development and toxin production. We have also deciphered how these two independent processes may be simultaneously regulated at the genetic level. These discoveries may ultimately allow us to develop properties in the crops that could together shut down fungal growth and consequently toxin production.
Technical Abstract: Aflatoxins (AFs), are secondary metabolites produced mainly by Aspergillus parasiticus and A. flavus. To study AF regulation, previously isolated non-toxigenic A. parasiticus sec- (for secondary metabolism minus) variants were genetically analyzed. In this study, we have used both classical and molecular genetic approaches to further analyze this highly intriguing class of non-toxigenic sec- variants. Specifically, we have focused our attention on 1) the behaviour of the sec- spores in parasexual crosses with respect to formation of heterokaryons and diploids, and 2) the aflR gene with respect to its DNA sequence and transcript levels in the sec- strains, using both traditional Northerns and the more sensitive RT-PCR technique to ascertain the degree of involvement of this regulatory gene in the formation of the sec- phenotype. In parasexual crossing, the sec- strains failed to form heterokaryons and diploids with other sec- strains. Heterokaryon test results suggested that involvement of cytoplasmic elements in the formation of sec- phenotype was unlikely. At the molecular level, the coding sequence of the sec- aflR (the only known positive regulator of AF pathway) was identical to that of their toxigenic sec- (for secondary metabolism plus) parents. However, the sec- aflR expression was 5 to 10-fold lower compared to that in the sec+ forms. RT-PCR analysis demonstrated that the AF pathway genes were expressed in the sec- forms but in trace amounts and in their unprocessed forms. Combined, these results suggest that aflR is necessary but not sufficient for aflatoxin production.