| Filamentous fungi have long been recognized to be a rich source of bioactive secondary metabolites. The genomic sequencing of several species of Aspergillus, however, has revealed that the number of secondary metabolite genes indicated in the genome far exceeds the number of products that have already been obtained in the laboratory, suggesting that many more metabolites await discovery. The challenge is to develop methods to uncover these hidden natural products. The work herein describes the discovery of several metabolites not previously observed from Aspergillus nidulans and the study of their biosynthesis in a post-genomic context. First, a group of polyketide-nonribosomal peptides known as the emericellamides were identified by using state-of-the-art detection techniques. Through efficient gene deletion analysis, a biosynthesis was proposed. Next, the same gene deletion technology was used to delete a gene involved in modulating chromatin architecture, with the idea that previously inaccessible sections of the genome would be available to the transcriptional machinery. Indeed, two groups of related compounds, monodictyphenone/emodins and F9775 A/B, were revealed. The polyketides F9775 A and B were also generated when the culture conditions were altered, indicating that the problem of unlocking a secondary metabolite can have multiple solutions. Gene deletions showed that a cluster of three genes were responsible for the formation of F9775 A/B, implying that other genes for these complex molecules exist elsewhere in the genome. Finally, the two major prenylated xanthones of A. nidulans, shamixanthone and emericellin, were previously identified, but two more were discovered, and genome-based deletion analysis uncovered much information about the four compounds' biosynthesis. Unprecedented, the genes for these metabolites were proven to be located in at least three distinct loci in the genome. Moreover, the two responsible prenyltransferase genes belong to a family known as fungal indole prenyltransferases, when the data here suggests that these enzymes in general may also catalyze the prenylation of polyketides. Overall, the results suggest that, contrary to the current paradigm, fungal secondary metabolite genes may not be wholly clustered in some instances, accenting the utility of genomics combined with rapid, powerful methods of targeting these dispersed genes. |
