Heat-Stable Antifungal Factor (HSAF) Biosynthesis in Lysobacter enzymogenes Is Controlled by the Interplay of Two Transcription Factors and a Diffusible Molecule.

Lysobacter enzymogenes is a Gram-negative, environmentally ubiquitous bacterium that produces a secondary metabolite, called heat-stable antifungal factor (HSAF), as an antifungal factor against plant and animal fungal pathogens. 4-Hydroxybenzoic acid (4-HBA) is a newly identified diffusible factor that regulates HSAF synthesis via L. enzymogenes LysR (LysRLe), an LysR-type transcription factor (TF). Here, to identify additional TFs within the 4-HBA regulatory pathway that control HSAF production, we reanalyzed the LenB2-based transcriptomic data, in which LenB2 is the enzyme responsible for 4-HBA production. This survey led to identification of three TFs (Le4806, Le4969, and Le3904). Of them, LarR (Le4806), a member of the MarR family proteins, was identified as a new TF that participated in the 4-HBA-dependent regulation of HSAF production. Our data show the following: (i) that LarR is a downstream component of the 4-HBA regulatory pathway controlling the HSAF level, while LysRLe is the receptor of 4-HBA; (ii) that 4-HBA and LysRLe have opposite regulatory effects on larR transcription whereby larR transcript is negatively modulated by 4-HBA while LysRLe, in contrast, exerts positive transcriptional regulation by directly binding to the larR promoter without being affected by 4-HBA in vitro; (iii) that LarR, similar to LysRLe, can bind to the promoter of the HSAF biosynthetic gene operon, leading to positive regulation of HSAF production; and (iv) that LarR and LysRLe cannot interact and instead control HSAF biosynthesis independently. These results outline a previously uncharacterized mechanism by which biosynthesis of the antibiotic HSAF in L. enzymogenes is modulated by the interplay of 4-HBA, a diffusible molecule, and two different TFs. [ABSTRACT FROM AUTHOR]