Abstract
A species-specific region, denoted SpG8-1b allowing hydroxycinnamic acids (HCAs) degradation is important for the transition between the two lifestyles (rhizospheric versus pathogenic) of the plant pathogen Agrobacterium fabrum. Indeed, HCAs can be either used as trophic resources and/or as induced-virulence molecules. The SpG8-1b region is regulated by two transcriptional regulators, namely, HcaR (Atu1422) and Atu1419. In contrast to HcaR, Atu1419 remains so far uncharacterized. The high-resolution crystal structures of two fortuitous citrate complexes, two DNA complexes and the apoform revealed that the tetrameric Atu1419 transcriptional regulator belongs to the VanR group of Pfam PF07729 subfamily of the large GntR superfamily. Until now, GntR regulators were described as dimers. Here, we showed that Atu1419 represses three genes of the HCAs catabolic pathway. We characterized both the effector and DNA binding sites and identified key nucleotides in the target palindrome. From promoter activity measurement using defective gene mutants, structural analysis and gel-shift assays, we propose N5,N10-methylenetetrahydrofolate as the effector molecule, which is not a direct product/substrate of the HCA degradation pathway. The Zn2+ ion present in the effector domain has both a structural and regulatory role. Overall, our work shed light on the allosteric mechanism of transcription employed by this GntR repressor.
Highlights
IntroductionA. fabrum possesses a species-specific region, denoted SpG8-1b (a region present in strains of this species but absent from other Agrobacterium species), located in the circular chromosome and responsible for hydroxycinnamic acids (HCAs) degradation such as ferulic acid, caffeic acid and pcoumaric acid (4,5) (Figure 1)
Agrobacterium fabrum has two lifestyles: it can interact with a large variety of plants as a rhizosphere inhabitant or as a pathogen when it harbors a tumor-inducing plasmid, and transfers a portion of this to the plant cells upon infection, resulting in the crown-gall disease (1-3).A. fabrum possesses a species-specific region, denoted SpG8-1b, located in the circular chromosome and responsible for hydroxycinnamic acids (HCAs) degradation such as ferulic acid, caffeic acid and pcoumaric acid (4,5) (Figure 1)
The transcriptional regulator Atu1419 is the second repressor of the HCAs degradation pathway
Summary
A. fabrum possesses a species-specific region, denoted SpG8-1b (a region present in strains of this species but absent from other Agrobacterium species), located in the circular chromosome and responsible for hydroxycinnamic acids (HCAs) degradation such as ferulic acid, caffeic acid and pcoumaric acid (4,5) (Figure 1). These latter compounds are common plant secondary metabolites being precursors of lignin incorporated into plant cell walls. We have previously showed that HCAs degradation via the SpG8-1b region interferes with virulence genes expression suggesting that this metabolic pathway is important for the transition between the two lifestyles (rhizospheric versus pathogenic) of Agrobacterium (11). Such a transition requires a fine-tune regulation of gene expression to express the appropriate genes at the right time (12-14)
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