Global Awakening of Cryptic Biosynthetic Gene Clusters in Burkholderia thailandensis

Abstract

Many bacteria encode biosynthetic proteins that produce a vast array of natural products. These compounds are often synthesized during host invasion as they function as virulence factors. In addition, such secondary metabolites have yielded numerous molecular scaffolds with pharmaceutical and clinical importance. The gene clusters that encode proteins responsible for synthesis of these compounds are typically silenced or “cryptic” under laboratory growth conditions, hampering discovery of novel lead compounds. We report here that MftR is a global repressor of secondary metabolite synthesis in Burkholderia thailandensis and that urate functions as a physiologically relevant inducer of gene expression. Biosynthetic gene clusters under MftR control include those associated with production of the antimicrobial bactobolins, the iron siderophore malleobactin, and the virulence factor malleilactone. MftR also controls additional genes associated with survival in a host environment, such as genes encoding components of the type III secretion system (T3SS) and proteins linked to anaerobic respiration. This observation not only has implications for understanding activation of gene regulatory networks during host invasion, but it also paves the way for isolation of novel therapeutic leads.

MarR Family Transcription Factors from Burkholderia Species: Hidden Clues to Control of Virulence-Associated Genes

“…Considering the plasticity of Burkholderia genomes and the variable genome sizes, such diversity of regulons is likely; for example, B. cenocepacia J2315 was isolated from a CF patient, and ϳ21% of its genome differs from other B. cenocepacia genomes, perhaps reflecting optimization for persistence in the CF lung (19). Among the annotated Burkholderia MarR homologs, only four (B. thailandensis MftR, BifR, and OhrR and B. pseudomallei TctR) have been characterized (64)(65)(66)(67)(68)(69).…”

“…This genomic locus (along with the two palindromes in the mftR-mftP intergenic region identified as MftR-binding sites [67]) is conserved in the closely related species B. mallei and B. pseudomallei, while only mftR and mftP (and the binding sites) are conserved in B. cenocepacia, and the entire locus is absent from B. xenovorans (Table 2). MftR is a negative regulator of both mftR and mftP-fenI, and binding of urate to MftR results in attenuation of DNA binding and upregulation of gene expression (64,66,67). Since urate is produced by host xanthine oxidase in response to bacterial infection, the implication is that MftR would be important for controlling gene expression after host colonization.…”

“…The absence of FenI results in clumping of bacterial cells in culture (64). Since FenI is predicted to be a glycosyl hydrolase, one possibility is that it may be involved in cleavage of the glycosidic bond between sugars in exopolysaccharides, thereby promoting detachment of cells.…”

“…A genome-wide expression analysis revealed that MftR controls a number of genes that are associated with survival in a host environment, genes that are also differentially expressed upon the addition of urate (64). For example, genes associated with survival under hypoxic conditions and the production of siderophores are upregulated in ΔmftR cells, whereas the large gene clusters that encode T3SS components and effectors are repressed.…”

Do Global Regulators Hold the Key to Production of Bacterial Secondary Metabolites?

“…Malleilactone (also identified as Burkholderic acid [14]), is a polyketide synthase (PKS)-derived cytotoxic product, produced by species in the B. pseudomallei complex (Bpc), and it has received much attention, as it has been shown to be essential for B. pseudomallei and B. thailandensis to cause infection in Caenorhabditis elegans (Figure 1). Malleilactone can also inhibit growth of Gram-positive bacteria and it is cytotoxic to cultured mammalian cells [15][16][17]. Proteins that are encoded by the mal gene cluster produce it (Table 1).…”