Comparison of the mechanism of transmembrane signaling in bacterial chemoreceptors and sensor kinases
Membrane-bound receptors transmit information from the cell exterior to the cell interior. Bacterial receptors capable of transmitting this information include sensor kinases, which control gene expression via response regulators, and methyl-accepting chemotaxis proteins (MCPs), which control rotation of the flagellar motor. These receptors, which have a similar general architecture and function, are predicted to share similar mechanisms of transmembrane signaling. The majority of such work has been conducted on MCPs. Our goal is to extend this work to the closely related sensor kinases by creating functional hybrid transducers. I show that a chimeric protein (Nart) that joins the periplasmic, ligandbinding domain of the sensor kinase NarX (nitrate/nitrite sensor) to the cytoplasmic signaling domain of the chemoreceptor Tar is capable of modulating flagellar rotation in response to both nitrate and nitrite. Consistent with the properties of NarX, our Nart elicits a stronger response to nitrate than to nitrite. Introduction of mutations into a highly conserved periplasmic region affects Nart signaling in a fashion that is consistent with the effects seen in NarX. I also present the first example of a substitution in a presumed ligand-binding domain that confers a reverse-signal phenotype for both nitrate and nitrite in Nart. These results support the hypothesis that the key aspects of transmembrane signaling are closely similar in homodimeric bacterial chemoreceptors and sensor kinases.