| Abstract
| - Fumonisins are polyketide-derived mycotoxins produced by the filamentous fungus Gibberellamoniliformis (anamorph Fusarium verticillioides). Wild-type strains of the fungus produce predominantlyfour B-series fumonisins, designated FB1, FB2, FB3, and FB4. Recently, a cluster of 15 putativefumonisin biosynthetic genes (FUM) was described in G. moniliformis. We have now conducted afunctional analysis of FUM13, a gene in the cluster that is predicted by amino acid sequence similarityto encode a short chain dehydrogenase/reductase (SDR). Mass spectrometric analysis of metabolitesfrom FUM13 deletion mutants revealed that they produce approximately 10% of wild-type levels ofB-series fumonisins as well as two previously uncharacterized compounds. NMR analysis revealedthat the new compounds are similar in structure to FB3 and FB4 but that they have a carbonyl functionrather than a hydroxyl function at carbon atom 3 (C-3). These results indicate that the FUM13 proteincatalyzes the reduction of the C-3 carbonyl to a hydroxyl group and are the first biochemical evidencedirectly linking a FUM gene to a specific reaction during fumonisin biosynthesis. The production oflow levels of FB1, FB2, FB3, and FB4, which have a C-3 hydroxyl, by the FUM13 mutants suggeststhat G. moniliformis has an additional C-3 carbonyl reductase activity but that this enzyme functionsless efficiently than the FUM13 protein. Keywords: Fumonisin; Gibberella moniliformis; Fusarium verticillioides; mycotoxin; gene cluster
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