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Biosynthesis of aflatoxin--the role of nutritional factors.

All mycotoxins, including aflatoxins, are a subclass of substances originated from secondary fungal metabolism. Secondary metabolism and metabolites constitute a wide range of biosynthetic pathways and products that have little in common except that, unlike primary metabolites, they are not used for growth. The production of secondary metabolites not only is strain-dependent but is influenced by the environmental and nutritional status of the producing organism and, in particular, may be regulated by growth limitation (Bu’Lock 1975). In batch cultures containing nutritionally-rich media, high levels of secondary metabolites are usually produced only after most of the cellular growth has already occurred. In a defined medium supporting only slow growth, however, some nutritional factor may be growthlimiting from the very start of cultivation, thus favouring production while slow growth is occurring (Demain 1986). Some of the more important mechanisms for control of primary metabolism are substrate induction, feedback repression and inhibition, catabolite repression and inhibition, and ATP (or energy charge) regulation. The regulation of primary metabolism by these and other mechanisms has been well studied (Caltrider and Niss 1966; Atkinson 1971). The pathways of secondary metabolism are regulated by many of the same processes that control primary metabolism (Drew and Demain 1977). The ways in which primary metabolites interact in secondary metabolism are evident on several levels, including roles as precursors and