Agrobacterium fabrum atu0526-Encoding Protein Is the Only Chemoreceptor That Regulates Chemoattraction toward the Broad Antibacterial Agent Formic Acid

Abstract

Soil-born plant pathogens, especially <i>Agrobacterium</i>, generally navigate their way to hosts through recognition of the root exudates by chemoreceptors. However, there is still a lack of appropriate identification of chemoreceptors and their ligands in <i>Agrobacterium</i>. Here, Atu0526, a sCache-type chemoreceptor from <i>Agrobacterium fabrum</i> C58, was confirmed as the receptor of a broad antibacterial agent, formic acid. The binding of formic acid to Atu0526 was screened using a thermo shift assay and verified using isothermal titration calorimetry. Inconsistent with the previously reported antimicrobial properties, formic acid was confirmed to be a chemoattractant to <i>A. fabrum</i> and could promote its growth. The chemotaxis of <i>A. fabrum</i> C58 toward formic acid was completely lost with the knock-out of <i>atu0526</i>, and regained with the complementation of the gene, indicating that Atu0526 is the only chemoreceptor for formic acid in <i>A. fabrum</i> C58. The affinity of formic acid to Atu0526^LBD significantly increased after the arginine at position 115 was replaced by alanine. However, in vivo experiments showed that the R115A mutation fully abolished the chemotaxis of <i>A. fabrum</i> toward formic acid. Molecular docking based on a predicted 3D structure of Atu0526 suggested that the arginine may provide "an anchorage" for formic acid to pull the minor loop, thereby forming a conformational change that generates the ligand-binding signal. Collectively, our findings will promote an understanding of sCache-type chemoreceptors and their signal transduction mechanism.