Old role with new feature: T2SS ATPase as a cyclic-di-GMP receptor to regulate antibiotic production

Cyclic di-GMP (c-di-GMP) is a crucial signaling molecule found extensively in bacteria, involved in the regulation of various physiological and biochemical processes such as biofilm formation, motility, and pathogenicity through binding to downstream receptors. However, the structural dissimilarity of c-di-GMP receptor proteins has hindered the discovery of many such proteins. In this study, we identified LspE, a homologous protein of the type II secretion system (T2SS) ATPase GspE in Lysobacter enzymogenes, as a receptor protein for c-di-GMP. We identified the more conservative c-di-GMP binding amino acid residues as K358 and T359, which differ from the previous reports, indicating that GspE proteins may represent a class of c-di-GMP receptor proteins. Additionally, we found that LspE in L. enzymogenes also possesses a novel role in regulating the production of the antifungal antibiotic HSAF. Further investigations revealed the critical involvement of both ATPase activity and c-di-GMP binding in LspE-mediated regulation of HSAF (Heat-Stable Antifungal Factor) production, with c-di-GMP binding having no impact on LspE's ATPase activity. This suggests that the control of HSAF production by LspE encompasses two distinct processes: c-di-GMP binding and the inherent ATPase activity of LspE. Overall, our study unraveled a new function for the conventional protein GspE of the T2SS as a c-di-GMP receptor protein and shed light on its role in regulating antibiotic production.IMPORTANCEThe c-di-GMP signaling pathway in bacteria is highly intricate. The identification and functional characterization of novel receptor proteins have posed a significant challenge in c-di-GMP research. The type II secretion system (T2SS) is a well-studied secretion system in bacteria. In this study, our findings revealed the ATPase GspE protein of the T2SS as a class of c-di-GMP receptor protein. Notably, we discovered its novel function in regulating the production of antifungal antibiotic HSAF in Lysobacter enzymogenes. Given that GspE may be a conserved c-di-GMP receptor protein, it is worthwhile for researchers to reevaluate its functional roles and mechanisms across diverse bacterial species.

A YajQ-LysR-like, cyclic di-GMP-dependent system regulating biosynthesis of an antifungal antibiotic in a crop-protecting bacterium, Lysobacter enzymogenes

YajQ, a binding protein of the universal bacterial second messenger cyclic di-GMP (c-di-GMP), affects virulence in several bacterial pathogens, including Xanthomonas campestris. In this bacterium, YajQ interacts with the transcription factor LysR. Upon c-di-GMP binding, the whole c-di-GMP-YajQ-LysR complex is found to dissociate from DNA, resulting in virulence gene regulation. Here, we identify a YajQ-LysR-like system in the bacterial biocontrol agent Lysobacter enzymogenes OH11 that secretes an antifungal antibiotic, heat-stable antifungal factor (HSAF) against crop fungal pathogens. We show that the YajQ homologue, CdgL (c-di-GMP receptor interacting with LysR) affects expression of the HSAF biosynthesis operon by interacting with the transcription activator LysR. The CdgL-LysR interaction enhances the apparent affinity of LysR to the promoter region upstream of the HSAF biosynthesis operon, which increases operon expression. Unlike the homologues CdgL (YajQ)-LysR system in X. campestris, we show that c-di-GMP binding to CdgL seems to weaken CdgL-LysR interactions and promote the release of CdgL from the LysR-DNA complex, which leads to decreased expression. Together, this study takes the YajQ-LysR-like system from bacterial pathogens to a crop-protecting bacterium that is able to regulate antifungal HSAF biosynthesis via disassembly of the c-di-GMP receptor-transcription activator complex.