Genetics and Biosynthesis of Fumonisins

Genetic and biochemical studies on fumonisin biosynthesis are fairly recent. No comprehensive biosynthetic pathway and genetic and enzymatic establishments like aflatoxins or trichothecenes are available currently. Fumonisin biosynthesis has been of significant research interest in the last few years (Desjardins and Proctor 1999; Proctor et al. 1999a; Proctor 2000). A linear 19- or 20-carbon chain is the fumonisin backbone that is substituted at various positions with hydroxyl, methyl, and tricarballylic acid moieties and an amino group at C-2 (Proctor 2000). Carbon labeling studies suggested that a PKS, alanine, methionine and other amino acids are involved in fumonisin synthesis (Desjardins et al. 1996a). Classical genetic analyses do provide some insight into the fumonisin biosynthesis and genetic control. Desjardins et al. (1996b) established that the genes responsible for fumonisin biosynthesis are closely linked on chromosome 1 when they studied on the FB1-non-producing mutant of G. fujikuroi MP-A. By complementation analysis in FB1-non-producing mutant of G. fujikuroi MP-A using fum5-containing cosmid clone, Proctor et al. (1999b) demonstrated that the fum5 is involved in fumonisin biosynthesis. The predicted amino acid sequence of fum5 showed high degree of similarity to fungal type I PKSs. Disruption and complementation analysis of the cloned G. fujikuroi gene, fum5, encoding a PKS, Proctor et al. (1999b) concluded that fumonisins are synthesized from polyketide rather than fatty acids. In order to identify additional genes that are potentially involved in fumonisin production near fum5, the DNA regions approximately 7-kb downstream and 15-kb upstream of this fum5 gene have been sequenced (Proctor 2000). A total of 8 ORFs, 5 ORFs in the upstream and 3 ORFs in the downstream have been identified. Based on sequence homologies to known genes and proteins, four of the ORFs appear to encode proteins that would be expected to be involved in fumonisin production. One of these ORFs encodes a putative cytochrome P-450 monooxygenase. Even though none of the fumonisin biosynthetic genes was isolated except fum5 for the PKS, classic genetic analysis using Gibberella fujikuroi (sexual stage of F. moniliforme) have identified four loci involved in fumonisin biosynthesis designated fum1, fum2, fum3, and fum4 (Desjardins et al. 1995; Desjardins et al. 1996a,b; Plattner et al. 1996; Seo et al. 2001). Studies indicated that the allele, fum1, might be involved in regulation of fumonisin production (Plattner et al. 1996); the allele, fum2, might be involved in hydroxylation of fumonisin at C-10 position (Desjardins et al. 1996b); the allele, fum3, might be involved in hydroxylation of fumonisin at C-5 position (Desjardins et al. 1996b); an additional UV generated mutant (uv26) has similar phenotype like fum3

(Proctor et al. 1997). Xu and Leslie (1996) mapped fum1 onto the chromosome 1 in F. moniliforme and the other fum loci were also mapped onto chromosome 1. Based on the recombination frequencies, a relative distance and linear relationship of the fum loci and some other gene loci have been established (Proctor 2000) to be in sequential order, ald1, fum1, fum3 (uv26), fum2, and OPA16.

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