Fungal-Associated Molecules Induce Key Genes Involved in the Biosynthesis of the Antifungal Secondary Metabolites Nunamycin and Nunapeptin in the Biocontrol Strain Pseudomonas fluorescens In5

• Published in: Applied and environmental microbiology Vol. 86; no. 21
• Main Authors: Christiansen, Line, Alanin, Katrine Skov, Phippen, Christopher B W, Olsson, Stefan, Stougaard, Peter, Hennessy, Rosanna C
• Format: Journal Article
• Language: English
• Published: United States American Society for Microbiology 15.10.2020
• Subjects: Antimicrobial Cationic Peptides - biosynthesis; Bacterial Proteins - biosynthesis; Biological control; Biological Control Agents - metabolism; Biosynthesis; Chemical synthesis; Environmental Microbiology; Fungi; Fungicides; Genes; Lipopeptides; Lipopeptides - biosynthesis; Low temperature; Metabolism; Metabolites; Phytotoxins; Promoter Regions, Genetic; Pseudomonas fluorescens; Pseudomonas fluorescens - metabolism; Secondary Metabolism; Secondary metabolites; Transcription factors; secondary metabolites; gene fusions; gene regulation; lipopeptides; Pseudomonas; natural antimicrobial products; biocontrol
• ISSN: 0099-2240; 1098-5336
• DOI: 10.1128/AEM.01284-20

In5 synthesizes the antifungal cyclic lipopeptides (CLPs) nunamycin and nunapeptin, which are similar in structure and genetic organization to the pseudomonas-derived phytotoxins syringomycin and syringopeptin. Regulation of syringomycin and syringopeptin is dependent on the two-component global regulatory system GacS-GacA and the SalA, SyrF, and SyrG transcription factors, which activate syringomycin synthesis in response to plant signal molecules. Previously, we demonstrated that a specific transcription factor, NunF, positively regulates the synthesis of nunamycin and nunapeptin in In5 and that the gene is upregulated by fungal-associated molecules. This study focused on further unravelling the complex regulation governing CLP synthesis in In5. Promoter fusions were used to show that the specific activator NunF is dependent on the global regulator of secondary metabolism GacA and is regulated by fungal-associated molecules and low temperatures. In contrast, GacA is stimulated by plant signal molecules leading to the hypothesis that is a hyphosphere-associated bacterium carrying transcription factor genes that respond to signals indicating the presence of fungi and oomycetes. Based on these findings, we present a model for how synthesis of nunamycin and nunapeptin is regulated by fungal- and oomycete-associated molecules. Cyclic lipopeptide (CLP) synthesis gene clusters in pseudomonads display a high degree of synteny, and the structures of the peptides synthesized are very similar. Accordingly, the genomic island encoding the synthesis of syringomycin and syringopeptin in pv. closely resembles that of In5, which contains genes coding for synthesis of the antifungal and anti-oomycete peptides nunamycin and nunapeptin, respectively. However, the regulation of syringomycin and syringopeptin synthesis is different from that of nunamycin and nunapeptin synthesis. While CLP synthesis in the plant pathogen pv. is induced by plant signal molecules, such compounds do not significantly influence synthesis of nunamycin and nunapeptin in In5. Instead, fungal-associated molecules positively regulate antifungal peptide synthesis in In5, while the synthesis of the global regulator GacA in In5 is positively regulated by plant signal molecules but not fungal-associated molecules.