Abstract
Aspergillus oryzae is traditionally used in East Asia for the production of food and brewing. In addition, it has been developed into a suitable host for the heterologous expression of natural product biosynthetic genes and gene clusters, enabling the functional analysis of the encoded enzymes. A. oryzae shares a 99.5% genome homology with Aspergillus flavus, but their secondary metabolomes differ significantly and various compounds unique to A. oryzae have been reported. While using A. oryzae as a host for heterologous expression experiments we discovered two new metabolites in extracts of A. oryzae M-2-3 with an unusual maleidride backbone, which were named oryzine A and B. Their structures were elucidated by high resolution mass spectrometry (HRMS) and nuclear magnetic resonance (NMR) analysis. Their structural relationships with known maleidrides implied involvement of a citrate synthase (CS) and a polyketide (PKS) or fatty acid synthase (FAS) in their biosynthesis. Analysis of the A. oryzae genome revealed a single putative biosynthetic gene cluster (BGC) consistent with the hypothetical biosynthesis of the oryzines. These findings increase knowledge of the chemical potential of A. oryzae and are the first attempt to link a novel product of this fungus with genomic data.
Highlights
Aspergillus oryzae is a filamentous fungus widely used in the food production industry of manyEast Asian countries to ferment soy beans as basis for soy sauces, miso and fermented bean paste, and for brewing of various beverages [1]
As part of a larger project involving the use of A. oryzae M-2-3 [18] as a host for the heterologous expression of engineered fungal polyketide synthase genes, we were interested in the biosynthesis of pretenellin A 1
This compound is produced by the tenellin synthetase (TENS) which is a hybrid polyketide synthase non-ribosomal peptide synthetase (PKS-NRPS) which works with a trans-acting enoyl reductase, encoded by tenC or dmbC [24,25]
Summary
Aspergillus oryzae is a filamentous fungus widely used in the food production industry of manyEast Asian countries to ferment soy beans as basis for soy sauces, miso and fermented bean paste, and for brewing of various beverages [1]. In combination with its advantageously limited set of native compounds A. oryzae has been successfully developed as a heterologous expression system for fungal secondary metabolites [2,3]. This system has been important in the discovery and functional analysis of various biosynthetic gene clusters (BGC) including xenovulenes, pleuromutilins, maleidrides and aphidicolin [4,5,6,7]. Whole genome sequencing of the RIB40 strain of A. oryzae and other Aspergillus species revealed a 99.5% similarity between the genomes of Aspergillus flavus and A. oryzae. It has been suggested that the latter is a domesticated variety of A. flavus, mainly differing in the lack of aflatoxin
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