title: Characteristics of the methylammonium/ammonium transport systems of the nitrogen-fixing cyanobacterium Anabaena 7120 (ATCC 27893).
abstract: NH4(+)-transport in Anabaena 7120 was studied using the NH4+ analogue, 14CH3NH3+. At pH 7, two energy-dependent NH4(+)-transport systems were detected in both N2- and NO3(-)-grown cells, but none in NH4(+)-grown cells. Both transport systems showed a low and a high affinity mode of operation depending on the substrate concentration. One of the transport systems showed Km values of 8 microM (Vmax = 1 nmole min-1mg-1protein) and 80 microM (Vmax = 7 nmole min-1mg-1protein), and was insensitive to L-methionine-DL-sulphoximine, a glutamate analogue and irreversible inhibitor of glutamine synthetase. The other transport system showed Km values of 2.5 microM (Vmax = 0.1 nmole min-1mg-1protein) and 70 microM (Vmax = 0.7 nmole min-1mg-1protein), and was sensitive to L-methionine-DL-sulphoximine. Intracellular accumulation of free 14CH3NH3+ showed a biphasic pattern in response to variation in external 14CH3NH3+ concentrations. A maximum intracellular concentration of 2.5 mM and 7.5 mM was reached in the external 14CH3NH3+ concentration range of 1-50 microM and 1-500 microM, respectively. At pH 9, an energy-independent diffusion of 14CH3NH2 leading to a higher intracellular accumulation and assimilation rate, than that at pH 7, was observed.
authors.forename: A N
authors.initials: AN
authors.lastname: Rai
authors.forename: R
authors.initials: R
authors.lastname: Prakasham
mesh_terms: D001692:Biological Transport; D000458:Cyanobacteria / Q000187:drug effects / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D008744:Methylamines / Q000493:pharmacokinetics*
chemical_list: D008744:Methylamines; C027451:methylamine