Control of Iron Regulation and Uptake by the ExpR/Sin Quorum Sensing System in Sinorhizobium Meliloti
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Sinorhizobium meliloti is a gram-negative soil bacterium that establishes a symbiotic association with the legume host Medicago sativa. The bacteria use the ExpR/Sin QS system, a cell-cell based communication mechanism, to invade the root nodules and fix nitrogen for the plant. The process of establishing symbiosis between legume and the bacteria requires an interplay of many factors; one of the most pivotal is iron. Studies in the past have shown that legumes involved in symbiosis have a greater requirement for iron and limiting the availability of iron has a tremendous impact on the efficiency of nodulation and nitrogen fixation. This is due to the fact that many of the key enzymes and proteins involved in symbiotic nitrogen fixation, such as nitrogenase use iron as a cofactor, and as a result there is a high demand for iron by the nitrogen-fixing bacteroids in the root nodules. Though iron is one of the most abundant transition metals on the Earth’s crust, at a physiological pH, it is both poorly soluble and unavailable. Therefore, microbes have adopted several strategies to obtain iron; one of the most efficient is the use of siderophores, diffusible molecules that are secreted under strict iron-limited conditions with a very high-affinity for the ferric (Fe⁺³) form of iron. S. meliloti produces rhizobactin 1021 as its predominant siderophore. The biosynthesis of rhizobactin 1021 is mediated by six genes arranged in the operon rhbABCDEF. Additionally, rhtX and rhtA; the genes that code for an outer membrane receptor and a permease, respectively, help in the recognition and transport of the iron-siderophore complexes across the membrane using the energy generated from the ExbBD-TonB complex. As in the case with other siderophores, the synthesis and release of rhizobactin 1021 is positively regulated by rhrA and negatively in the presence of iron by the rhizobial iron uptake regulator RirA. Quantitative Real Time-PCR analyses conducted in our laboratory showed differential expression of the genes involved in the synthesis, transport, and regulation of rhizobactin 1021 in a wild-type strain compared to the QS mutants that lacked either the sinI or the expR component of the QS system. Symbiosis studies conducted on plants inoculated with a QS capable strain vs. plants inoculated with a QS mutant showed that a wild-type is far more efficient in invading root nodules under iron-limiting conditions. These results suggested that the presence of an intact ExpR/Sin QS system might help S. meliloti to cope with iron scarcity. Therefore, in this current study, we set out to understand the possible role of the ExpR/Sin QS in siderophore synthesis and regulation and its influence on plant root nodulation.