In , a Gram-positive soil bacterium widely used in the industrial production of amino acids, two genes encoding (putative) ammonium uptake carriers have been described. The isolation of was the first report of the sequence of a gene encoding a bacterial ammonium uptake system combined with the characterization of the corresponding protein. Recently, a second gene, , with so far unknown function, was isolated. The isolation of this gene and the suggestion of a new concept for ammonium acquisition prompted the reinvestigation of ammonium transport in . In this study it is shown that Amt mediates uptake of (methyl)ammonium into the cell with high affinity and strictly depending on the membrane potential. As shown by the determination of at different pH values, ammonium/methylammonium, but not ammonia/methylamine, are substrates of Amt. AmtB exclusively accepts ammonium as a transport substrate. In addition, hints of another, until now unknown, low-affinity, ammonium-specific uptake system were found.


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