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Bacteria have multiple K+ uptake systems.Escherichia coli , 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.Azorhizobium caulinodans ORS571, a rhizobium that forms nitrogen-fixing nodules on the stems and roots ofSesbania rostrata , also has three types of K+ uptake systems. Through phylogenetic analysis, we found thatA. caulinodans has two genes homologous totrkG andtrkH , designatedtrkI andtrkJ . We also found thattrkI is adjacent totrkA in the genome and these two genes are transcribed as an operon; however,trkJ is present at a distinct locus. Our results demonstrated thattrkAI ,trkJ , andkup were expressed in the wild-type stem nodules, whereaskdpFABC was not. Interestingly, Δkup and Δkup ΔkdpA mutants formed Fix– nodules, while the Δkup ΔtrkA ΔtrkI ΔtrkJ mutant formed Fix+ nodules, suggesting that with the additional deletion of Trk system genes in the Δkup mutant, Fix+ nodule phenotypes were recovered.kdpFABC of the Δkup ΔtrkJ mutant was expressed in stem nodules, but not in the free-living state, under high-K+ conditions. However,kdpFABC of the Δkup ΔtrkA ΔtrkI ΔtrkJ mutant was highly expressed even under high-K+ conditions. The cytoplasmic K+ levels in the Δkup ΔtrkA ΔtrkI mutant, which did not expresskdpFABC under high-K+ conditions, were markedly lower than those in the Δkup ΔtrkA ΔtrkI ΔtrkJ mutant. Taking all these results into consideration, we propose that TrkJ is involved in the repression ofkdpFABC in response to high external K+ concentrations and that the TrkAI system is unable to function in stem nodules.IMPORTANCE 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 ofEscherichia coli 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 ofAzorhizobium caulinodans acts as a sensor for the extracellular K+ concentration and that high extracellular K+ concentrations repress the expression of KdpFABC via TrkJ.

applied and environmental microbiology

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