Submitted to: Gene
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/24/2000
Publication Date: N/A
Interpretive Summary: Aflatoxins are natural toxins produced by two common molds, Aspergillus flavus and A. parasiticus. When the fungi invade crops and produce aflatoxins, they render the crops unsalable. The cancer causing property of aflatoxins has stimulated many scientists to study in details how these toxins are produced. Our efforts are focused on the understanding aflatoxin biosynthesis by characterizing the genes and enzymes involved in this process. It is well known that aflatoxin synthesis is affected by the type of carbon (sugar) source required for fungal growth and development. Simple sugars such as glucose and fructose support toxin synthesis, whereas complex sugars or peptides such as starch and peptone do not. In this report, we present the identification of a cluster of four genes in the aflatoxigenic fungal strain A. parasiticus. These genes are possibly involved in supplying sugar substrate for aflatoxin production, since the sugar cluster resides near a cluster of genes that govern the chemical steps for aflatoxin formation. Additionally, at least one of the genes for sugar metabolism activates at the same time as the aflatoxin pathway genes during the growth of the fungus. The discovery of this group of genes may give insight into the co-regulation of aflatoxin production and sugar utilization. This information will be used to devise strategies for control of aflatoxin contamination in pre-harvest crops and provide a tool for the elimination of aflatoxin contamination in food and feed.
Technical Abstract: Aflatoxins are extremely potent carcinogens produced by the fungi Aspergillus parasiticus and A. flavus in food and feed. In our study on understanding molecular regulation of aflatoxin synthesis, we have cloned the 70 kb aflatoxin pathway gene cluster. At one end of this cluster we have cloned a group of four genes that constitute a gene cluster related to osugar utilization in A. parasiticus. (1) a gene, named sugR, containing a C6-zinc-finger motif, has homology to the amyR gene in A. sojae and Bacillus subtilis for the regulation of expression of the sugar utilization pathway genes; (2) a gene, hxtA, encodes a membrane protein for uptake of hexoses; (3) a third gene, glcA, has homology to genes that encode a-1,6-glucosidases for the release of glucose from starch and the side chain of amylopectin; (4) a fourth gene, nadA, has homology to genes that encode NADH oxidase. The sugar utilization pathway gene cluster and the previously identified aflatoxin pathway gene cluster are situated next to each other on the chromosome in the fungal genome. These two clusters are separated by a 5 kb DNA spacer in which no known ORF has been identified. On the other boundary of the sugar cluster a 4 kb DNA region separates it from the next closest functional gene group. For the first time, we have identified this well-defined, unique gene cluster in A. parasiticus. No similar gene cluster has ever been identified so far in any other organisms. The expression of the hxtA gene is concurrent with the aflatoxin pathway cluster genes in aflatoxin conducive medium as demonstrated by Northern blot analysis and RT-PCR experiments. The possible co-regulation of the two clusters genetically and the biological effects of this sugar utilization cluster on aflatoxin production are discussed.