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Chemotaxis systems enable microbes to sense their immediate environment, moving towards beneficial stimuli and away from those that are harmful. In an effort to better understand the chemotaxis system of Sinorhizobium meliloti , a symbiont of the legume alfalfa, the cellular stoichiometries of all ten chemotaxis proteins in S. meliloti were determined. A combination of quantitative immunoblot and mass spectrometry revealed that the protein stoichiometries in S. meliloti varied greatly from those in Escherichia coli and Bacillus subtilis . To compare protein ratios to other systems, values were normalized to the central kinase CheA. All S. meliloti chemotaxis proteins exhibited increased ratios to varying degrees. The ten-fold higher molar ratio of adaptor proteins CheW1 and CheW2 to CheA might result in the formation of rings in the chemotaxis array that only consist of CheW instead of CheA and CheW in a 1:1 ratio. We hypothesize that the higher ratio of CheA to the main response regulator CheY2 is a consequence of the speed-variable motor in S. meliloti , instead of a switch-type motor. Similarly, proteins involved in signal termination are far more abundant in S. meliloti , which utilizes a phosphate-sink mechanism based on CheA retro-phosphorylation to inactivate the motor response regulator versus CheZ-catalyzed dephosphorylation as in E. coli and B. subtilis . Finally, the abundance of CheB and CheR, which regulate chemoreceptor methylation, was increased when compared to CheA, indicative of variations in the adaptation system of S. meliloti . Collectively, these results mark significant differences in the composition of bacterial chemotaxis systems. IMPORTANCE The symbiotic soil bacterium Sinorhizobium meliloti contributes greatly to host-plant growth by fixing atmospheric nitrogen. The provision of nitrogen as ammonium by S. meliloti leads to increased biomass production of its legume host alfalfa and diminishes the use of environmentally harmful chemical fertilizers. To better understand the role of chemotaxis in host-microbe interaction, a comprehensive catalogue of the bacterial chemotaxis system is vital, including its composition, function, and regulation. The stoichiometry of chemotaxis proteins in S. meliloti has very few similarities to the systems in E. coli and B. subtilis . In addition, total amounts of proteins are significantly lower. S. meliloti exhibits a chemotaxis system distinct from known models by incorporating new proteins as exemplified by the phosphate sink mechanism.
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Chemoreceptors in Sinorhizobium meliloti require minimal pentapeptide tethers to provide adaptational assistance
Abstract Sensory adaptation in bacterial chemotaxis is mediated by posttranslational modifications of methyl‐accepting chemotaxis proteins (MCPs). InEscherichia coli, the adaptation proteins CheR and CheB tether to a conserved C‐terminal receptor pentapeptide. Here,we investigated the function of the pentapeptide motif (N/D)WE(E/N)F inSinorhizobium melilotichemotaxis. Isothermal titration calorimetry revealed stronger affinity of the pentapeptides to CheR and activated CheB relative to unmodified CheB. Strains with mutations of the conserved tryptophan in one or all four MCP pentapeptides resulted in a significant decrease or loss of chemotaxis to glycine betaine, lysine, and acetate, chemoattractants sensed by pentapeptide‐bearing McpX and pentapeptide‐lacking McpU and McpV, respectively. Importantly, we discovered that the pentapeptide mediates chemotaxis when fused to the C‐terminus of pentapeptide‐lacking chemoreceptors via a flexible linker. We propose that adaptational assistance and a threshold number of available sites enable the efficient docking of adaptation proteins to the chemosensory array. Altogether, these results demonstrate thatS. melilotieffectively utilizes a pentapeptide‐dependent adaptation system with a minimal number of tethering units to assist pentapeptide‐lacking chemoreceptors and hypothesize that the higher abundance of CheR and CheB inS. meliloticompared toE. coliallows for ample recruitment of adaptation proteins to the chemosensory array.
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- Award ID(s):
- 2128232
- PAR ID:
- 10524997
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Molecular Microbiology
- Volume:
- 122
- Issue:
- 1
- ISSN:
- 0950-382X
- Format(s):
- Medium: X Size: p. 50-67
- Size(s):
- p. 50-67
- Sponsoring Org:
- National Science Foundation
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