title: Sinorhizobium meliloti chemotaxis to quaternary ammonium compounds is mediated by the chemoreceptor McpX.
abstract: The bacterium Sinorhizobium meliloti is attracted to seed exudates of its host plant alfalfa (Medicago sativa). Since quaternary ammonium compounds (QACs) are exuded by germinating seeds, we assayed chemotaxis of S. meliloti towards betonicine, choline, glycine betaine, stachydrine and trigonelline. The wild type displayed a positive response to all QACs. Using LC-MS, we determined that each germinating alfalfa seed exuded QACs in the nanogram range. Compared to the closely related nonhost species, spotted medic (Medicago arabica), unique profiles were released. Further assessments of single chemoreceptor deletion strains revealed that an mcpX deletion strain displayed little to no response to these compounds. Differential scanning fluorimetry showed interaction of the isolated periplasmic region of McpX (McpXPRand McpX34-306) with QACs. Isothermal titration calorimetry experiments revealed tight binding to McpXPRwith dissociation constants (Kd) in the nanomolar range for choline and glycine betaine, micromolar Kdfor stachydrine and trigonelline and a Kdin the millimolar range for betonicine. Our discovery of S. meliloti chemotaxis to plant-derived QACs adds another role to this group of compounds, which are known to serve as nutrient sources, osmoprotectants and cell-to-cell signalling molecules. This is the first report of a chemoreceptor that mediates QACs taxis through direct binding.
authors.affiliation: Department of Biological Sciences, Life Sciences I, Virginia Tech, Blacksburg, VA, 24061, USA.
authors.forename: Benjamin A
authors.initials: BA
authors.lastname: Webb
authors.affiliation: Department of Biological Sciences, Life Sciences I, Virginia Tech, Blacksburg, VA, 24061, USA.
authors.forename: K
authors.initials: K
authors.lastname: Karl Compton
authors.affiliation: Department of Biological Sciences, Life Sciences I, Virginia Tech, Blacksburg, VA, 24061, USA.
authors.forename: Rafael
authors.initials: R
authors.lastname: Castañeda Saldaña
authors.affiliation: Department of Biological Sciences, Life Sciences I, Virginia Tech, Blacksburg, VA, 24061, USA.
authors.forename: Timofey D
authors.initials: TD
authors.lastname: Arapov
authors.affiliation: Department of Biochemistry, Life Sciences I, Virginia Tech, Blacksburg, VA, 24061, USA.
authors.forename: W
authors.initials: W
authors.lastname: Keith Ray
authors.affiliation: Department of Biochemistry, Life Sciences I, Virginia Tech, Blacksburg, VA, 24061, USA.
authors.forename: Richard F
authors.initials: RF
authors.lastname: Helm
authors.affiliation: Department of Biological Sciences, Life Sciences I, Virginia Tech, Blacksburg, VA, 24061, USA.
authors.forename: Birgit E
authors.initials: BE
authors.lastname: Scharf
mesh_terms: D001426:Bacterial Proteins / Q000378:metabolism; D001622:Betaine / Q000378:metabolism; D002628:Chemoreceptor Cells / Q000378:metabolism; D002633:Chemotaxis; D002794:Choline / Q000378:metabolism; D000455:Medicago sativa / Q000382:microbiology; D000644:Quaternary Ammonium Compounds / Q000378:metabolism*; D016962:Sinorhizobium meliloti / Q000378:metabolism*; D013559:Symbiosis / Q000502:physiology
chemical_list: D001426:Bacterial Proteins; D000644:Quaternary Ammonium Compounds; D001622:Betaine; D002794:Choline