Search for: All records

ABSTRACT Agrobacterium fabrum is a phytopathogen that causes crown gall disease. In the rhizosphere, it encounters plant exudates, some of which are toxic, such as 4-hydroxybenzaldehyde (4HBA). Others, including 4-hydroxybenzoate (4HB), participate in the induction of virulence genes.A. fabrum encodes the transcription factor PecS, which has been reported to enhance bacterial fitness in the rhizosphere. The gene encoding PecS is divergent from pecM, which encodes an efflux pump. PecS represses both pecS and pecM, as evidenced by increased expression in the presence of the PecS ligand urate and by elevated pecM expression in a pecS disruption strain. We report here that the expression ofpecM is induced selectively by 4HBA. Expression of genes encoding enzymes involved in the degradation of 4HB is induced by both 4HBA and 4HB, as expected; however, overexpression ofpecM attenuates the induction by 4HBA, suggesting that 4HBA is a substrate for PecM. Consistent with this inference, untargeted metabolomics shows that 4HBA accumulates intracellularly whenpecM is disrupted. Analysis of PecS by thermal stability assay and DNase I footprinting suggests that 4HBA is not a ligand for PecS. Taken together, our data suggest that 4HBA is a substrate for PecM.IMPORTANCEPlant roots secrete a number of compounds that may be toxic to bacteria residing in the surrounding soil. One such bacterium is Agrobacterium fabrum, which infects plants and induces tumor formation. We show here that an A. fabrum strain in which the efflux pump PecM has been disrupted accumulates 4-hydroxybenzaldehyde, and that this plant root exudate induces the expression of pecM. Our data suggest that PecM and PecS, a transcription factor that regulates pecM expression, both function to promote A. fabrum fitness in the rhizosphere. As a competitive advantage in the rhizosphere is a prerequisite for subsequent plant infection, our data contribute to a more complete understanding of the A. fabrum infection process. 

ABSTRACT Bacterial plant pathogens adjust their gene expression programs in response to environmental signals and host-derived compounds. This ensures that virulence genes or genes encoding proteins, which promote bacterial fitness in a host environment, are expressed only when needed. Such regulation is in the purview of transcription factors, many of which belong to the ubiquitous multiple antibiotic resistance regulator (MarR) protein family. PecS proteins constitute a subset of this large protein family. PecS has likely been distributed by horizontal gene transfer, along with the divergently encoded efflux pump PecM, suggesting its integration into existing gene regulatory networks. Here, we discuss the roles of PecS in the regulation of genes associated with virulence and fitness of bacterial plant pathogens. A comparison of phenotypes and differential gene expression associated with the disruption of pecS shows that functional consequences of PecS integration into existing transcriptional networks are highly variable, resulting in distinct PecS regulons. Although PecS universally binds to the pecS-pecM intergenic region to repress the expression of both genes, binding modes differ. A particularly relaxed sequence preference appears to apply for Dickeya dadantii PecS, perhaps to optimize its integration as a global regulator and regulate genes ancestral to the acquisition of pecS-pecM. Even inducing ligands for PecS are not universally conserved. It appears that PecS function has been optimized to match the unique regulatory needs of individual bacterial species and that its roles must be appreciated in the context of the regulatory networks into which it was recruited. 

CysB is a member of the large bacterial LysR-type transcriptional regulator (LTTR) protein family. Like the majority of LTTRs, CysB functions as a homotetramer in which each subunit has an N-terminal winged-helix-turn-helix (wHTH) DNA-binding domain connected to an effector-binding domain by a helical hinge region. CysB is best known for its role in regulating the expression of genes associated with sulfur uptake and biosynthesis of cysteine in Gram-negative species such as Escherichia coli and Salmonella enterica. Activation of CysB target genes generally requires the effector N-acetyl-L-serine, which derives from an intermediate in the cysteine biosynthetic pathway. Here, we outline the established roles of CysB in controlling the cysteine regulon, complemented with an interpretation of DNA binding modes inspired by the recently published structure of full-length CysB that is consistent with the ‘sliding dimer’ model proposed for many LTTRs. Notably, CysB orthologs have been described for which N-acetyl-L-serine does not appear to be required as an effector, and CysB regulons frequently include genes that are not directly related to sulfur assimilation and cysteine biosynthesis. Examples include hslJ, which encodes a predicted membrane protein involved in novobiocin resistance in E. coli, and pqsR, encoding a transcriptional regulator involved in Pseudomonas Quinolone Signal production and virulence in Pseudomonas aeruginosa. These data suggest that CysB orthologs have diverged to ensure optimal function and incorporation in distinct gene regulatory networks.