The unicellular alga Chlamydomonas reinhardtii and the bacterium Pseudomonas protegens serve as a model to study the interactions between photosynthetic and heterotrophic microorganisms. P. protegens secretes the cyclic lipopeptide orfamide A that interferes with cytosolic Ca 2+ homeostasis in C. reinhardtii resulting in deflagellation of the algal cells. Here, we studied the roles of additional secondary metabolites secreted by P. protegens using individual compounds and co-cultivation of algae with bacterial mutants. Rhizoxin S2, pyrrolnitrin, pyoluteorin, 2,4-diacetylphloroglucinol (DAPG) and orfamide A all induce changes in cell morphology and inhibit the growth of C. reinhardtii. Rhizoxin S2 exerts the strongest growth inhibition, and its action depends on the spatial structure of the environment (agar versus liquid culture). Algal motility is unaffected by rhizoxin S2 and is most potently inhibited by orfamide A (IC 50 = 4.1 μM). Pyrrolnitrin and pyoluteorin both interfere with algal cytosolic Ca 2+ homeostasis and motility whereas high concentrations of DAPG immobilize C. reinhardtii without deflagellation or disturbance of Ca 2+ homeostasis. Co-cultivation with a regulatory mutant of bacterial secondary metabolism (ΔgacA) promotes algal growth under spatially structured conditions. Our results reveal how a single soil bacterium uses an arsenal of secreted antialgal compounds with complementary and partially overlapping activities.
Genomes of cyanobacteria feature a variety of cryptic biosynthetic pathways for complex natural products, but the peculiarities limiting the discovery and exploitation of the metabolic dark matter are not well understood. Here we describe the discovery of two cell density-dependent chemical mediators, nostoclide and nostovalerolactone, in the symbiotic model strain Nostoc punctiforme, and demonstrate their pronounced impact on the regulation of specialized metabolism. Through transcriptional, bioinformatic and labeling studies we assigned two adjacent biosynthetic gene clusters to the biosynthesis of the two polyketide mediators. Our findings provide insight into the orchestration of specialized metabolite production and give lessons for the genomic mining and high-titer production of cyanobacterial bioactive compounds.
Cyanobakterielle Genome weisen eine Vielzahl unbekannter Biosynthesewege für komplexe Naturstoffe auf, allerdings sind die Besonderheiten, die die Entdeckung und Nutzung unbekannter Stoffwechselwege einschränken, nicht hinreichend verstanden. Hier beschreiben wir die Entdeckung zweier von der Zelldichte abhängiger chemischer Mediatoren, Nostoclid und Nostovalerolacton, welche im symbiotischen Modellstamm Nostoc punctiforme vorkommen und demonstrieren ihren ausgeprägten Einfluss auf die Regulation des spezialisierten Stoffwechsels. Mithilfe von Transkriptions‐, Bioinformatik‐ und Markierungsstudien konnten wir die Biosynthese der beiden Polyketid‐Mediatoren zwei benachbarten biosynthetischen Genclustern zuordnen. Unsere Ergebnisse geben einen Einblick in die Produktionsmechanismen spezialisierter Metabolite und liefern wertvolle Erkenntnisse für das Genomic Mining und die Hochtiter‐Produktion von cyanobakteriellen bioaktiven Substanzen.
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