DNA Distortion by the Burkholderia thailandensis Transcriptional Regulator FusR2 Is Abolished by Fusaric Acid Binding.

Fusaric acid is a toxic metabolite produced by several Fusarium species, which causes wilt disease in many plants. Among other functions, fusaric acid allows the fungus to outcompete soil bacteria. Understanding mechanisms by which bacteria mitigate the toxic effects of fusaric acid is therefore of interest in terms of controlling Fusarium wilt. The soil bacterium Burkholderia thailandensis encodes a predicted fusaric acid-binding membrane transporter belonging to the FusC2 family. The fusC2 gene is annotated as part of an operon, also encoding an isochorismatase (gene named isoC) and a member of the multiple antibiotic resistance (MarR) family of transcription factors. Isochorismatase converts isochorismate to the antifungal compound 2,3-dihydroxybenzoate. We show here that the transcription factor, which we named FusR2, binds specifically to the promoter of the fusR2-isoC-fusC2 operon and that FusR2 induces marked changes in DNA conformation, as evidenced by hypersensitive DNase I cleavage sites. Fusaric acid induces the expression of fusR2-isoC-fusC2, and it binds directly to FusR2, as shown by thermal shift assays. While the presence of fusaric acid is compatible with DNA binding by FusR2, it eliminates the hypersensitive DNA cleavage. We propose that FusR2 imposes a DNA conformation, which adversely affects the ability of RNA polymerase to bind, whereas fusaric acid binding to FusR2 results in an altered DNA binding mode, in which the RNA polymerase can compete with FusR2 for DNA binding to initiate transcription. By this mechanism, fusaric acid induces the expression of genes encoding both an efflux pump and an enzyme involved in the production of an antifungal metabolite.