Summary
Lysobacter enzymogenes is a non‐flagellated, soil proteobacterium that secretes a diffusible antibiotic known as heat‐stable antifungal factor (HSAF) to kill nearby fungi for food. The genome of the model strain OH11 encodes a homologous Wsp system, which is generally deployed by flagellated bacteria to achieve flagella‐dependent outputs via a c‐di‐GMP‐FleQ complex, in which c‐di‐GMP is a ubiquitous dinucleotide second messenger and FleQ is a transcription factor (TF). Here, we show that the Wsp system in the non‐flagellated OH11 participates in a unique c‐di‐GMP‐dependent signalling pathway and forms a WspR‐CdgL binary complex to alter HSAF production, in which WspR and CdgL act as a c‐di‐GMP diguanylate cyclase (DGC) and a non‐TF binding protein respectively. We found that the phosphorylation of WspR activates its DGC activity and enhances c‐di‐GMP production while inhibiting HSAF biosynthesis. The phosphorylation of WspR also plays a key role in weakening WspR‐CdgL binding and HSAF generation. Interestingly, c‐di‐GMP binding to CdgL did not seem to induce the disassociation of the WspR‐CdgL complex. These observations, along with our earlier findings, lead us to propose a model in which L. enzymogenes re‐programs the Wsp system via c‐di‐GMP signalling to regulate HSAF biosynthesis for the benefit of ecological adaptation.
Bacteria interact with fungi in a variety of ways to inhibit fungal growth, while the underlying mechanisms remain only partially characterized. The plantbeneficial Bacillus and Pseudomonas species are well-known antifungal biocontrol agents, whereas Lysobacter are far less studied. Members of Lysobacter are easy to grow in fermenters and are safe to humans, animals and plants. These environmentally ubiquitous bacteria use a diverse arsenal of weapons to prey on other microorganisms, including fungi and oomycetes. The small molecular toxins secreted by Lysobacter represent long-range weapons effective against filamentous fungi. The secreted hydrolytic enzymes act as intermediaterange weapons against non-filamentous fungi. The contact-dependent killing devices are proposed to work as short-range weapons. We describe here the structure, biosynthetic pathway, action mode and applications of one of the best-characterized longrange weapons, the heat-stable antifungal factor (HSAF) produced by Lysobacter enzymogenes. We discuss how the flagellar type III secretion system has evolved into an enzyme secretion machine for the intermediate-range antifungal weapons. We highlight an intricate mechanism coordinating the production of the long-range weapon, HSAF and the proposed contact-dependent killing device, type VI secretion system. We also overview the regulatory mechanisms of HSAF production involving specific transcription factors and the bacterial second messenger c-di-GMP.
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Graphical abstract
Lysobacter enzymogenes
, a robust bacterial warrior uses a “mobile-attack” antifungal strategy comprised of twitching motility powered by the type IV pilus and antifungal weapons. This strategy is controlled by a “busy” transcription factor Clp via multiple previously uncharacterized mechanisms, including binding with DNA in unique modes, interacting directly or responding to diverse small-molecule chemicals, as well as associating specifically with structurally-distinct proteins.
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