Multiplicity of ammonium uptake systems in Corynebacterium glutamicum: role of Amt and AmtB

In Corynebacterium glutamicum, a Gram-positive soil bacterium widely used i n the industrial production of amino acids, two genes encoding (putative) a mmonium uptake carriers have been described. The isolation of amt was the f irst report of the sequence of a gene encoding a bacterial ammonium uptake system combined with the characterization of the corresponding protein. Rec ently, a second amt gene, amtB, with so far unknown function, was isolated. The isolation of this gene and the suggestion of a new concept for ammoniu m acquisition prompted the reinvestigation of ammonium transport in C. glut amicum. In this study it is shown that Amt mediates uptake of (methyl)ammon ium into the cell with high affinity and strictly depending on the membrane potential. As shown by the determination of K-m at different ph values, am monium/methylammonium, but not ammonia/methylamine, are substrates of Amt. AmtB exclusively accepts ammonium as a transport substrate. In addition, hi nts of another, until now unknown, low-affinity, ammonium-specific uptake s ystem were found.