Functional analysis of the polyketide synthase genes in the filamentous fungus Gibberella zeae (Anamorph (Fusarium graminearum)

The fungus Gibberella zeae Schw. [Schwein.] Petch (anamorph Fusarium graminearum Schwabe) is a major causal agent of the disease Fusarium Head Blight when it infects cereals such as wheat, oats, barley and corn. The fungus produces several mycotoxins both during infection of the grain and storage of that infected grain. The myxotoxins characterized to date include zearalenone, fusarin C and aurofusarin which are of polyketide origin and deoxynivalenol which is a trichothecene.; Polyketides are a class of secondary metabolites that exhibit a vast diversity of form and function. In fungi, these compounds are produced by large, multi-domain enzymes classified as Type I polyketide synthases (PKS). In the past, most PKS genes were identified because they produced a compound of interest. However, with the recent advent of the genomic era, it is possible to clone and characterize the entire suite of PKS genes within a genome, contributing to the overall analysis of the total polyketide potential of an organism. Although PKSs are composed of several highly conserved domains, the extensive diversity of the PK products arise from the number and kind of domains present on the enzyme and the type of starter and extender units selected by the enzyme. In this study we identified all fifteen PKS genes within the genome of the filamentous fungus G. zeae. We succeeded in identifying five genes responsible for producing the previously identified compounds zearalenone, aurofusarin, fusarin C and the black perithecial pigment by analysis of disrupted mutants of all fifteen genes. A comprehensive expression study revealed several interesting expression patterns. However we were unable to detect expression of one of the PKS genes under any of seventeen conditions tested. This is the first study to genetically characterize a complete set of PKS genes from a single organism.; Zearalenone is a mycotoxin of worldwide economic and health importance. It is most commonly found as a contaminant in stored corn and wheat grain and has chronic estrogenic effects on mammals. Zearalenone has been known to be a polyketide, derived from the sequential addition of multiple acetate units by a polyketide synthase (PKS). However, the genetics of zearalenone biosynthesis and the identification of associated modifying enzymes have not been elucidated. Herein, we describe the cloning of two genes, designated ZEA1 and ZEA2, that encode the polyketide synthases responsible for zearalenone production in G. zeae. Disruption of either of these genes results in the loss of zearalenone production by the resulting mutant. ZEA1 and ZEA2 are transcribed divergently from a common promoter region. Analysis of this region gives clues to the regulation of these genes. Examination of the region of the chromosome involved in zearalenone biosynthesis shows the presence of two possible regulatory proteins. A putative mechanism for the synthesis of zearalenone by the two PKSs is proposed.