UREA UPTAKE BY THE MARINE BACTERIUM DELEYA-VENUSTA HG1

The uptake (transport and metabolism) of urea was studied in a strain of the marine bacterium Deleya venusta, measuring the uptake of [C-14]urea in vivo and the urease reaction in vitro. Urea uptake in vivo was sodium-dependent and exhibited a K(m) value of 1.4 muM for urea, a broad pH optimum between pH 6.0 and 8.5, a distinct temperature optimum at 35-degrees-C and a requirement for energy. Urease activity in vitro exhibited a K(m) value of 0.86 mM for urea and showed maximum activities at pH 8.5 and 60-degrees-C; the enzyme was neither dependent on the presence of sodium, nor inhibited by metabolic inhibitors. Synthesis of the urea uptake system was subject to nitrogen control; ammonium resulted in a repression of the system, whereas high uptake rates were observed after growth with nitrate or incubation of the cells in the absence of a nitrogen source. The uptake reaction in vivo, but not the urease activity in vitro, was decreased greatly in the presence of ammonium. This inhibition was relieved by methionine sulphoximine (MSX), a potent inhibitor of glutamine synthetase; in mutant strains impaired in this enzyme no inhibition of urea uptake by ammonium was observed. These results suggest that glutamine formed from ammonium rather than ammonium itself regulates urea uptake activity in D. venusta.