Abstract
The uptake of the radioactive ammoniumanalogue 14C-methylammonium1 was measured in heterotrophically grown cells of Alcaligenes eutrophus H16 in order to study the mechanism of NH 4 + uptake. MA gradients of up to 200 were built up by a substrate-specific and energy-dependent system which showed a Km of 35–111 μM and a Vmax of 0.4–1.8 nmol MA/min per mg protein. The involved carrier exhibited a higher affinity towards NH 4 + than towards CH3NH 3 + indicating that ammonium rather than MA was its natural substrate. Cold shock with hypotonic but not with hypertonic solutions caused the efflux of almost the entire accumulated MA. Osmotic shock did not affect the uptake reaction, suggesting that no periplasmic binding proteins were involved. Indirect observations indicate the membrane potential as driving force for MA uptake. High rates of uptake were observed in cells grown under nitrogen deficiency or with nitrate as nitrogen source. The uptake rate decreased during growth at high ammonium concentrations indicating that biosynthesis of nitrogenous compounds was supported by passive diffusion of NH3. The data suggest that the formation of the carrier is subject to “nitrogen control”.
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