Chemotactic-signal transducers respond to changes in the concentration of attractants and repellents in the environment, transduce a signal from the outside to the inside of the cell, and facilitate sensory adaptation through the variation of the level of methylation. McpQ recognizes specifically citrate and citrate/metal(2+) complexes. Binds citrate/metal(2+) complexes with higher affinity than free citrate, and mediates preferentially chemotaxis toward citrate/metal(2+) complexes. Deletion of the gene causes very strong reduction in the chemotaxis toward citrate/Mg(2+) complexes. Belongs to the methyl-accepting chemotaxis (MCP) protein family.