|KIM, JONG HEON|
Submitted to: Applied Microbiology and Biotechnology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 15, 2005
Publication Date: May 22, 2005
Citation: Kim, J.H., Campbell, B.C., Molyneux, R.J., Mahoney, N.E., Chan, K.L., Yu, J., Wilkinson, J.R., Cary, J.W., Bhatnagar, D., Cleveland, T.E. 2005. Examination of fungal stress response genes using saccharomyces cerevisiae as a model system: targeting genes affecting aflatoxin biosynthesis by Aspergillus Flavus Link. Applied Microbiology and Biotechnology. 67(6):807-815. Interpretive Summary: Aflatoxin is a cancer-causing agent produced by certain fungi. These fungi can sometimes infect certain types of crops. As a result of this infection, harvested agricultural products can become contaminated with aflatoxin. This contamination is not only a food-safety problem, but is also an international trade issue because of very strict low levels for aflatoxin contamination allowed by importing countries. We have discovered some natural compounds that can prevent the fungus from making aflatoxin. In this paper, we outline how this prevention works. The compounds appear to alleviate a stress, called oxidative stress, from the fungus. This stress is something that occurs when the fungus infects plant tissue. This stress normally triggers the fungus to start to make aflatoxin. We also discovered some of the genes of the fungus that may be involved in this stress response using new methods. Knowledge of which genes of the fungus to target for preventing aflatoxin synthesis will help us to devise methods of controlling aflatoxin biosynthesis, or perhaps synthesis of other toxins made by other types of fungi.
Technical Abstract: Hydrolysable tannins containing gallic acid moieties and certain other phenolic acids are antioxidants that possess potent antiaflatoxigenic activity against Aspergillus flavus Link. The antiaflatoxigenic activity of these compounds is initiated upstream from the aflatoxin biosynthetic pathway. To determine the basis for the antiaflatoxigenic activity, Saccharomyces cerevisiae was used as a model fungal system. A broad spectrum of singular gene deletion mutants of S. cerevisiae was exposed to hydrogen peroxide (H2O2) in order to examine phenotype response to oxidative stress. Treatment of some mutants with gallic and other phenolic acids (e.g., caffeic acid) during H2O2 exposure indicated these compounds alleviated oxidative stress. Orthologs of 43 of the S. cerevisiae genes involved in gene regulation, signal transduction (e.g., sho1, hog1, etc.) and antioxidation (e.g., ctt1, cta1, etc.) were identified in A. flavus using an A. flavus Expressed Sequence Tag (EST) library. Functional complementation of one of the identified genes associated with oxidative stress, mitochondrial superoxide dismutase, supported use of S. cerevisiae deletion mutants to gain insights on the functional genomics of A. flavus. The likelihood of oxidative stress triggering aflatoxin biosynthesis in A. flavus and antioxidants, such as gallic acid, reducing this stress response is discussed.