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Abstract
Chemotaxis enables bacteria to move toward more favorable environmental conditions. We observed chemotaxis toward boric acid by Ralstonia pseudosolanacearum Ps29. At higher concentrations, the chemotactic response of R. pseudosolanacearum toward boric acid was comparable to or higher than that toward L-malate, indicating that boric acid is a strong attractant for R. pseudosolanacearum. Chemotaxis assays under different pH conditions suggested that R. pseudosolanacearum recognizes B(OH)3 (or B(OH3) + B(OH)4−) but not B(OH)4− alone. Our previous study revealed that R. pseudosolanacearum Ps29 harbors homologs of all 22R. pseudosolanacearum GMI1000 mcp genes. Screening of 22 mcp single-deletion mutants identified the RS_RS17100 homolog as the boric acid chemoreceptor, which was designated McpB. The McpB ligand-binding domain (LBD) was purified in order to characterize its binding to boric acid. Using isothermal titration calorimetry, we demonstrated that boric acid binds directly to the McpB LBD with a KD (dissociation constant) of 5.4 µM. Analytical ultracentrifugation studies revealed that the McpB LBD is present as a dimer that recognizes one boric acid molecule.
Highlights
Chemotaxis, a universal phenomenon in motile bacteria, involves sensing of chemical gradients and subsequent navigation toward/away from more favorable/unfavorable environmental conditions[1]
Careful review of the experimental procedure revealed that R. pseudosolanacearum exhibited much stronger responses to HEPES buffer stored in a borosilicate glass bottle for an extended time than to generally used buffer stored in plastic bottles, and these responses were reproducible
There was no significant difference between the responses to 5 mM silicate and the control (HEPES buffer stored in a plastic tube), but R. pseudosolanacearum Ps29 exhibited a significantly stronger response to 0.5 mM boric acid than to HEPES buffer (P < 0.05 by Student’s t test)
Summary
Chemotaxis, a universal phenomenon in motile bacteria, involves sensing of chemical gradients and subsequent navigation toward/away from more favorable/unfavorable environmental conditions[1]. Ralstonia solanacearum is a gram-negative, motile plant pathogenic bacterium that causes bacterial wilt in a number of economically important crops, including tomato, potato, eggplant, tobacco, and banana[16, 17]. This soil-borne bacterium usually enters plant roots through wounds, the root tips, and secondary root emergence points, eventually invading the xylem vessels and spreading to the aerial parts of the plant[18]. Pseudomonas strains exhibited no response to HEPES buffer, whereas R. pseudosolanacearum exhibited weak but significant chemotaxis toward HEPES buffer and occasionally even exhibited strong chemotaxis to the “negative” control We investigated this phenomenon in detail and found that R. pseudosolanacearum was attracted to boric acid. We describe chemotaxis toward boric acid by R. pseudosolanacearum and the identification and characterization of its chemotaxis sensor
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