McpQ is a specific citrate chemoreceptor that responds preferentially to citrate/metal ion complexes

Chemoreceptors are at the beginning of chemosensory pathways that mediate chemotaxis. Pseudomonas putida KT2440 is predicted to have 27 chemoreceptors, most of which uncharacterized. We have previously identified McpS as chemoreceptor for Krebs cycle intermediates. Citrate is primarily present in the environment as metal complex, which, however, is not recognized by McpS. We show here that the McpS paralogue McpQ recognizes specifically citrate and citrate/metal²⁺ complexes. The McpQ ligand binding domain (McpQ-LBD) binds citrate/metal²⁺ complexes with higher affinity than citrate. McpQ-LBD is present in a monomer-dimer equilibrium and citrate and particularly citrate/Mg²⁺ binding stabilize the dimer. The bacterium showed much stronger responses to citrate/Mg²⁺ than to citrate and mcpQ inactivation caused a dramatic reduction in chemotaxis. Responses to Krebs cycle intermediates are thus mediated by the broad range McpS and McpQ that responds specifically to an intermediate not recognized by McpS. Interesting parallels exist to the paralogous amino acid chemoreceptors of Pseudomonas aeruginosa and Bacillus subtilis. Whereas one paralogue recognizes most amino acids, the remaining paralogue binds specifically one of the few acids not recognized by the broad range receptors. Therefore, chemotaxis to compound families by the concerted action of broad and narrow range receptors may represent a general mechanism.