ABSTRACTLysobacter enzymogenesis 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 viaL. enzymogenesLysR (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 LysRLeis the receptor of 4-HBA; (ii) that 4-HBA and LysRLehave opposite regulatory effects onlarRtranscription wherebylarRtranscript is negatively modulated by 4-HBA while LysRLe, in contrast, exerts positive transcriptional regulation by directly binding to thelarRpromoter without being affected by 4-HBAin 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 LysRLecannot interact and instead control HSAF biosynthesis independently. These results outline a previously uncharacterized mechanism by which biosynthesis of the antibiotic HSAF inL. enzymogenesis modulated by the interplay of 4-HBA, a diffusible molecule, and two different TFs.IMPORTANCEBacteria use diverse chemical signaling molecules to regulate a wide range of physiological and cellular processes. 4-HBA is an “old” chemical molecule that is produced by diverse bacterial species, but its regulatory function and working mechanism remain largely unknown. We previously found that 4-HBA inL. enzymogenescould serve as a diffusible factor regulating HSAF synthesis via LysRLe. Here, we further identified LarR, an MarR family protein, as a second TF that participates in the 4-HBA-dependent regulation of HSAF biosynthesis. Our results dissected how LarR acts as a protein linker to connect 4-HBA and HSAF synthesis, whereby LarR also has cross talk with LysRLe. Thus, our findings not only provide fundamental insight regarding how a diffusible molecule (4-HBA) adopts two different types of TFs for coordinating HSAF biosynthesis but also show the use of applied microbiology to increase the yield of the antibiotic HSAF by modification of the 4-HBA regulatory pathway inL. enzymogenes.
Antibacterial effect of the adhering human Lactobacillus acidophilus strain LB.
