Geoffrey Turner

The filamentous fungus Aspergillus niger is widely exploited by the fermentation industry for the production of enzymes and organic acids, particularly citric acid. We sequenced the 33.9-megabase genome of A. niger CBS 513.88, the ancestor of currently used enzyme production strains. A high level of synteny was observed with other aspergilli sequenced. Strong function predictions were made for 6,506 of the 14,165 open reading frames identified. A detailed description of the components of the protein secretion pathway was made and striking differences in the hydrolytic enzyme spectra of aspergilli were observed. A reconstructed metabolic network comprising 1,069 unique reactions illustrates the versatile metabolism of A. niger. Noteworthy is the large number of major facilitator superfamily transporters and fungal zinc binuclear cluster transcription factors, and the presence of putative gene clusters for fumonisin and ochratoxin A synthesis.

A gene encoding a putative dimodular nonribosomal peptide synthetase (NRPS) was identified within a gene cluster of Aspergillus fumigatus, a species reported to produce fumitremorgins and other prenylated alkaloids. The gene was deleted and overexpressed in the genome reference strain Af293, and was also expressed in the naïve host Aspergillus nidulans, which lacks the equivalent gene cluster. While neither fumitremorgins nor the dipeptide brevianamide F (cyclo-L-Trp-L-Pro), an early intermediate, were detected in wild-type and deletion strains of A. fumigatus, brevianamide F accumulated in fungal cultures following increased expression of the NRPS gene in both A. fumigatus and A. nidulans. We conclude that the gene Afu8g00170, named ftmA, encodes the NRPS brevianamide synthetase. Brevianamide F is the precursor of a variety of fungal prenylated alkaloids with biological activity, including fumitremorgins A, B and C and tryprostatin B.

The genome sequence of Aspergillus fumigatus revealed the presence of a single hybrid polyketide synthase-non-ribosomal peptide synthetase (PKS/NRPS) gene that is present within a cluster of five genes suggestive of its involvement in secondary metabolism. Here, we present evidence that it is required for the biosynthesis of pseurotin A, a compound with an unusual heterospirocyclic gamma-lactam structure. We have confirmed that the genome reference strain A. fumigatus Af293 produces pseurotin A, a compound previously reported to be a competitive inhibitor of chitin synthase and an inducer of nerve-cell proliferation. Deletion or overexpression of the PKS/NRPS gene psoA in A. fumigatus leads to the absence or accumulation of pseurotin A, respectively; this indicates that this gene is essential for the biosynthesis of pseurotin A. It is likely that the first product of psoA is converted to pseurotin A by the products of other genes in this cluster.