This review examines the molecular biology of aflatoxin biosynthesis, toxins produced by Aspergillus species that contaminate crops and pose significant human and animal health risks. Aflatoxin B1 is highly mutagenic and carcinogenic, with the FDA setting strict regulatory limits. The aflatoxin biosynthetic pathway involves approximately 17 enzymes catalyzing sequential reactions from acetate and malonate units to the final toxic product. Researchers have successfully cloned several pathway genes including nor-1, ver-1, avm8, and omt-1 using genetic complementation and reverse genetics approaches. Key findings include the discovery of an aflatoxin gene cluster on a single chromosome with genes physically organized in an order reflecting enzyme reaction sequence. The regulatory gene aflR, encoding a zinc cluster-containing transcription factor, controls expression of pathway genes during the idiophase of fungal growth. Gene duplication of ver-1 and aflR in A. parasiticus may explain its higher toxin production stability compared to A. flavus. Understanding these molecular mechanisms offers potential strategies for eliminating aflatoxin from the food chain and controlling this important fungal pathogen.

Key findings

• Aflatoxin biosynthesis involves approximately 17 enzymes organized in a clustered gene arrangement on a single chromosome, with genes physically ordered similarly to their catalytic sequence
• The regulatory gene aflR encodes a zinc cluster-containing transcription factor that positively regulates aflatoxin pathway gene expression during fungal idiophase
• Gene duplication of ver-1 and aflR in A. parasiticus correlates with higher and more stable aflatoxin production compared to A. flavus, which lacks this duplication
• Multiple genes have been successfully cloned using genetic complementation (nor-1, ver-1, avm8) and reverse genetics approaches based on purified enzymes
• Aflatoxin B1's carcinogenicity depends on a 2,3 double bond in the difuran moiety that makes it susceptible to cytochrome P450 activation into DNA-damaging epoxides