ARS | Publication request: Aflatoxin-like Gene Clusters and How They Evolved

Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: January 19, 2008
Publication Date: April 1, 2009
Citation: Ehrlich, K., Yu, J. 2009. Aflatoxin-like Gene Clusters and How They Evolved. In: Rai, M., Varma, A. (Eds). Mycotoxins in Food, Feed and Bioweapons. Springer-Verlag Berlin Heidelberg. p. 65-75.

Interpretive Summary: Some fungal molds like A. flavus and A. parasiticus produce aflatoxins. They are toxic and tumor-inducing natural agents. Polyketides are early precursors of aflatoxins. They are also called secondary metabolites because it was thought that their synthesis began after the secession of primary metabolism and that such molecules were not required for survivial. Others are precursors of fungal pigments. Many are toxic to plants, animals, or soil microorganisms. Their toxicity often is used defensively by the fungus against competing organisms, or offensively, by facilitating invasion of plants. The best studied polyketides are known collectively as aflatoxins (AF). In this review, we will discuss current knowledge concerning the AF gene cluster and how such clusters may have evolved. This information will help better understanding the fungal biology and toxin formation for reducing and eliminating aflatoxin contamination.

Technical Abstract: Aflatoxins are toxic and carcinogenic polyketide, bis-furan metabolites produced by some Aspergillus species. Biosynthesis requires at least 22 enzymatic steps starting with hexanolyCoA. Biosynthetic proteins are encoded by genes in a 70 kb subtelomeric gene cluster. In this review, we discuss various aspects of the molecular biology of aflatoxin biosynthesis gene clusters, including the functions of the genes involved, and how they came to be clustered. The phylogenetic evidence supports the conclusion that the gene clusters for fungal metabolites allow rapid adaptation to selective pressures. Once formed, the gene clusters appear to be quite stable, even when adaptive pressures change or are removed. Based on the types of biosynthetic steps, the role of gene duplicates, and comparisons to related clusters from other aflatoxigenic and non-aflatoxigenic species, such gene clusters probably represent regulatory islands in the subtelomeric region of the chromosome.