Rapid Evolution of an Iron Acquisition System in Bradyrhizobium japonicum
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Bradyrhizobium japonicum USDA 110 does not synthesize ferric-chelating compounds, termed siderophores, to sequester iron from environment. Here, we show that selectivity for ferric-chelate utilization is limited primarily to the outer membrane and thus can be readily modified. Spontaneous gain-of-function mutations in outer membrane transporter genes of B. japonicum are sufficient to confer on cells the ability to use synthetic or natural iron siderophores. Moreover, growth on natural or synthetic chelators required the cytoplasmic membrane ferrous (Fe 2+ ) iron transporter FeoB, suggesting that iron is both dissociated from the chelate and reduced to the ferrous form within the periplasm prior to cytoplasmic entry. The data suggest rapid adaptation to environmental iron is facilitated by facile mutation of selective outer membrane transporter genes and by non-selective uptake components that do not require mutation to accommodate new iron sources. Furthermore, low iron stress resulted in over a five-fold increase in mutation frequency within B. japonicum cells, however, that stress did not favor adaptive mutations. The ability to readily evolve to acquire new ferric-chelates may facilitate B. japonicum to compete and cooperate in a microbial community.