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

作者：Siarot Lowela; Toyazaki Hiroki; Hidaka Makoto; Kurumisawa Keigo; Hirakawa Tomoki; Morohashi Kengo; Aono Toshihiro^*

来源：Applied and Environmental Microbiology, 2017, 83(19): UNSP e01197.

DOI：10.1128/AEM.01197-17

• potassium transport
• rhizobium
• symbiosis

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 the two constitutive systems, the TrkAG/TrkAH and Kup systems. Azorhizobium caulinodans ORS571, a rhizobium that forms nitrogen-fixing nodules on the stems and roots of Sesbania rostrata, also has three types of K+ uptake systems. Through phylogenetic analysis, we found that A. caulinodans has two trkG/H homologous genes, designated as trkI and trkJ. We also found that trkI is adjacent to trkA in the genome and these two genes are transcribed as an operon, however, trkJ is present at a distinct locus. Our results demonstrated that trkAI, trkJ, and kup were expressed in the wild type stem nodules, whereas kdpFABC was not. Interestingly, Delta kup and Delta kup Delta kdpA mutants formed Fix nodules while Delta kup Delta trkA Delta trkI Delta trkJ formed Fix(+) nodules suggesting that with the additional deletion of Trk system genes in Delta kup, Fix(+) nodule phenotypes were recovered. The Delta kup Delta trkJ mutant kdpFABC was expressed in stem nodules, but not in the free-living state under high K+ conditions. However, the Delta kup Delta trkA Delta trkI Delta trkJ mutant kdpFABC was highly expressed even under high K+ conditions. The cytoplasmic K+ levels in the Delta kup Delta trkA Delta trkI mutant, which did not express kdpFABC under high K+ conditions, were markedly lower than those in the Delta kup Delta trkA Delta trkI Delta trkJ mutant. Taking all these results into consideration, we propose that TrkJ is involved in the repression of kdpFABC 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 of Escherichia 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 of Azorhizobium caulinodans acts as a sensor for the extracellular K+ concentration and that high extracellular K+ concentrations repress the expression of KdpFABC via TrkJ.

• 出版日期2017-10