Abstract
Seed contamination with polyketide mycotoxins such as sterigmatocystin (ST) produced by Aspergilli is a worldwide issue. The ST biosynthetic pathway is well-characterized in A. nidulans, but regulatory aspects related to the carbon source are still enigmatic. This is particularly true for lactose, inasmuch as some ST production mutant strains still synthesize ST on lactose but not on other carbon substrates. Here, kinetic data revealed that on d-glucose, ST forms only after the sugar is depleted from the medium, while on lactose, ST appears when most of the carbon source is still available. Biomass-specified ST production on lactose was significantly higher than on d-glucose, suggesting that ST formation may either be mediated by a carbon catabolite regulatory mechanism, or induced by low specific growth rates attainable on lactose. These hypotheses were tested by d-glucose limited chemostat-type continuous fermentations. No ST formed at a high growth rate, while a low growth rate led to the formation of 0.4 mg·L−1 ST. Similar results were obtained with a CreA mutant strain. We concluded that low specific growth rates may be the primary cause of mid-growth ST formation on lactose in A. nidulans, and that carbon utilization rates likely play a general regulatory role during biosynthesis.
Highlights
Aflatoxins (AF) are among the most carcinogenic natural substances known to date and the most important mycotoxins
The velvet gene of A. nidulans is involved in the regulation of a variety of cellular processes such as asexual and sexual development as well as secondary metabolism [17]
A single base pair mutation at the full length veA start codon called veA1 was described to impair ST production [18]. To see whether this is the case under the conditions of this study, two veA1 mutant strains were tested for ST production and compared to a wild‐type strain (Figure 2 and Table 3)
Summary
Aflatoxins (AF) are among the most carcinogenic natural substances known to date and the most important mycotoxins. They are produced by a large diversity of ascomycetous fungal species, mainly from the genus Aspergillus [1]. These fungi can contaminate cereal crops and other staple commodities before harvest or during storage, leading to huge economic losses worldwide, and occasional famine in tropical countries [2]. ST is the penultimate intermediate in the biosynthesis of AF in A. flavus; in several other fungi including the model
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