A Novel Regulatory Pathway for K Uptake in the Legume Symbiont Azorhizobium caulinodans in Which TrkJ Represses the Operon at High Extracellular K Concentrations.

Bacteria have multiple K uptake systems. , for example, has three types of K uptake systems, which include the low-K-inducible KdpFABC system and two constitutive systems, Trk (TrkAG and TrkAH) and Kup. ORS571, a rhizobium that forms nitrogen-fixing nodules on the stems and roots of , also has three types of K uptake systems. Through phylogenetic analysis, we found that has two genes homologous to and , designated and We also found that is adjacent to in the genome and these two genes are transcribed as an operon; however, is present at a distinct locus. Our results demonstrated that , , and were expressed in the wild-type stem nodules, whereas was not. Interestingly, Δ and Δ Δ mutants formed Fix nodules, while the Δ Δ Δ Δ mutant formed Fix nodules, suggesting that with the additional deletion of Trk system genes in the Δ mutant, Fix nodule phenotypes were recovered. of the Δ Δ mutant was expressed in stem nodules, but not in the free-living state, under high-K conditions. However, of the Δ Δ Δ Δ mutant was highly expressed even under high-K conditions. The cytoplasmic K levels in the Δ Δ Δ mutant, which did not express under high-K conditions, were markedly lower than those in the Δ Δ Δ Δ mutant. Taking all these results into consideration, we propose that TrkJ is involved in the repression of in response to high external K concentrations and that the TrkAI system is unable to function in stem nodules. K is a major cytoplasmic cation in prokaryotic and eukaryotic cells. Bacteria have multiple K uptake systems to control the cytoplasmic K levels. In many bacteria, the K uptake system KdpFABC is expressed under low-K conditions. For years, many researchers have argued over how bacteria sense K concentrations. Although KdpD of is known to sense both cytoplasmic and extracellular K concentrations, the detailed mechanism of K sensing is still unclear. In this study, we propose that the transmembrane TrkJ protein of acts as a sensor for the extracellular K concentration and that high extracellular K concentrations repress the expression of KdpFABC via TrkJ.