Mutationally altered signal output in the Nart (NarX-Tar) hybrid chemoreceptor.

Signal-transducing proteins that span the cytoplasmic membrane transmit information about the environment to the interior of the cell. In bacteria, these signal transducers include sensor kinases, which typically control gene expression via response regulators, and methyl-accepting chemoreceptor proteins, which control flagellar rotation via the CheA kinase and CheY response regulator. We previously reported that a chimeric protein (Nart) that joins the ligand-binding, transmembrane, and linker regions of the NarX sensor kinase to the signaling and adaptation domains of the Tar chemoreceptor elicits a repellent response to nitrate and nitrite. As with NarX, nitrate evokes a stronger response than nitrite. Here we show that mutations targeting a highly conserved sequence (the P box) in the periplasmic domain alter chemoreception by Nart and signaling by NarX similarly. In particular, the G51R substitution converts Nart from a repellent receptor into an attractant receptor for nitrate. Our results underscore the conclusion that the fundamental mechanism of transmembrane signaling is conserved between homodimeric sensor kinases and chemoreceptors. They also highlight the plasticity of the coupling between ligand binding and signal output in these systems.