Deletion and Overexpression of the AnOTAbzip Gene, a Positive Regulator of Ochratoxin A Biosynthesis in Aspergillus niger

Metarhizium spp. encode an ochratoxin cluster and a high efficiency ochratoxin-degrading amidohydrolase revealed by genomic analysis

INTRODUCTION

Ochratoxins (OTs) are worldwide regulated mycotoxins contaminating a variety of food-environment and agro-environment. Several Aspergillus and Pencillium species synthesize OTs from a six-gene biosynthetic gene cluster (BGC) to produce the highly toxic final product OTA. Although many studies on OTA-degrading enzymes were performed, high efficiency enzymes with strong stability are extremely needed, and the OTA degrading mechanism is poorly understood.

OBJECTIVES

The study aimed to explore the OT-degradation enzyme and investigate its degradation mechanisms in Metarhizium, which contain an OT biosynthetic gene cluster.

METHODS

Phylogenomic relationship combined with RNA expression analysis were used to explore the distribution of OT BGC in fungi. Bioactivity-guided isolation and protein mass spectrometry were conducted to trace the degrading enzymes in Metarhizium spp., and the enzymes were heterologously expressed in E. coli and verified by in vitro assays. Structure prediction and point mutation were performed to reveal the catalytic mechanism of MbAmh1.

RESULTS

Beyond Aspergillus and Pencillium species, three species of the distant phylogenetic taxon Metarhizium contain an expressed OT-like BGC but lack an otaD gene. Unexpectedly, no OT BGC products were found in some Metarhizium species. Instead, Metarhizium metabolized both OTA and OTB to their non-toxic degradation products. This activity of M. brunneum was attributed to an intracellular hydrolase MbAmh1, which was tracked by bioactivity-guided proteomic analysis combined with in vitro reaction. Recombinant MbAmh1 (5 μg/mL) completely degraded 1 μg/mL OTA within 3 min, demonstrating a strong degrading ability towards OTA. Additionally, MbAmh1 showed considerable temperature adaptability ranging from 30 to 70 °C and acidic pH stability ranging from 4.0 to 7.0. Identification of active sites supported the crucial role of metal iron for this enzymatic reaction.

CONCLUSION

These findings reveal different patterns of OT synthesis in fungi and provide a potential OTA degrading enzyme for industrial applications.

Fumonisin B1 Biosynthesis Is Associated with Oxidative Stress and Plays an Important Role in Fusarium proliferatum Infection on Banana Fruit

Fungal response to oxidative stress during infection on postharvest fruit is largely unknown. Here, we found that hydrogen peroxide (H₂O₂) treatment inhibited the growth of Fusarium proliferatum causing crown rot of banana fruit, confirmed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observation. H₂O₂ exposure increased endogenous reactive oxygen species (ROS) and fumonisin B1 (FB1) production in F. proliferatum, possibly by modulating FUM or ROS-related gene expression. Importantly, H₂O₂ treatment inhibited F. proliferatum growth in vivo but induced FB1 accumulation in banana peel. Finally, we constructed the FpFUM21 deletion mutant (ΔFpfum21) of F. proliferatum that was attenuated in FB1 biosynthesis and less tolerant to oxidative stress. Moreover, the ΔFpfum21 strain was less virulent compared to the wild type (WT) due to the inability to induce FB1 production in the banana host. These results suggested that FB1 biosynthesis is associated with oxidative stress in F. proliferatum and contributes to fungal infection on banana fruit.

A Zn(II)(2)Cys(6) transcription factor MPsGeI suppresses pigment biosynthesis in Monascus

High-quality natural edible pigments known as monascus pigments (MPs) are widely used in food, medicine, and chemical industries as active functional ingredients. At the transcriptional level, the expression of MPs genes are tightly controlled, limiting their productivity and color value. Hitherto our understanding of the regulation of expression of MPs genes has been rather limited. Here, we describe a pathway-specific Zn(II)(2)Cys(6) transcription factor involved in the MPs biosynthetic cluster named MPsGeI, which encodes a 813-amino-acid protein with six introns. Expression of all MPs biosynthetic genes and accumulation of MPs were remarkably increased in ΔMPsGeI strain, and MPs production was significantly reduced in MPsGeI over-expressing strain. Results clearly demonstrated that MPsGeI negatively regulates MPs accumulation via transcriptional regulation of MPs biosynthetic genes, and plays a central repressive role in MPs' biosynthesis. Transcriptomic analyses revealed that MPsGeI disruptant regulated higher concentrations of precursors flowing to pigment and resulted in accumulation of a large amount of red MPs in hyphae. This work offers an efficient method for increasing MPs's productivity and color value and provides novel insights into the regulatory mechanisms of fungal cellular processes, which will assist to enhance MPs production and application.

AnAzf1 acts as a positive regulator of ochratoxin A biosynthesis in Aspergillus niger

Aspergillus niger produces genotoxic and carcinogenic ochratoxin A (OTA) that severely threatens human and animal health. Transcription factor Azf1 is essential in regulating fungal cell development and primary metabolism. However, its effect and mechanism on secondary metabolism are unclear. Here, we characterized and deleted a Azf1 homolog gene, An15g00120 (AnAzf1), in A. niger, which completely blocked OTA production, and repressed the OTA cluster genes, p450, nrps, hal, and bzip at the transcriptional level. The results indicated that AnAzf1 was a positive regulator of OTA biosynthesis. Transcriptome sequencing results showed that the AnAzf1 deletion significantly upregulated antioxidant genes and downregulated oxidative phosphorylation genes. Enzymes involved in reactive oxygen species (ROS) scavenging, including catalase (CAT) and peroxidase (POD) were increased, and the corresponding ROS levels were decreased. Upregulation of genes (cat, catA, hog1, and gfd) in the MAPK pathway and downregulation of genes in iron homeostasis were associated with decreased ROS levels, linking the altered MAPK pathway and iron homeostasis to lower ROS levels caused by AnAzf1 deletion. Additionally, enzymes including complex I (NADH-ubiquinone oxidoreductase), and complex V (ATP synthase), as well as ATP levels, were significantly decreased, indicating impaired oxidative phosphorylation caused by the AnAzf1-deletion. During lower ROS levels and impaired oxidative phosphorylation, OTA was not produced in ∆AnAzf1. Together, these results strongly suggested that AnAzf1 deletion blocked OTA production in A. niger by a synergistic interference of ROS accumulation and oxidative phosphorylation. KEY POINTS: • AnAzf1 positively regulated OTA biosynthesis in A. niger. • Deletion of AnAzf1 decreased ROS levels and impaired oxidative phosphorylation. • An altered MAPK pathway and iron homeostasis were associated with lower ROS levels.