This study examined urea uptake mechanisms in the marine bacterium Deleya venusta strain HG1, isolated from North Sea surface waters. The bacterium transported urea via an energy-dependent permease system with high affinity (Km = 1.4 µM), distinct from the subsequent urease-catalyzed hydrolysis. Urea uptake required sodium, exhibited optimal activity between pH 6.0-8.5 and at 35°C, and was inhibited by metabolic inhibitors, indicating dependence on electrochemical ion gradients rather than direct ATP hydrolysis. Synthesis of the uptake system was subject to nitrogen control, being repressed by ammonium and derepressed under nitrogen starvation or with nitrate. Intracellular glutamine, formed by glutamine synthetase from ammonia, inhibited uptake activity in living cells. This was demonstrated using methionine sulfoximine, a glutamine synthetase inhibitor, and confirmed through isolation of glutamine synthetase-deficient mutants that no longer showed ammonium-dependent inhibition of urea uptake. The high affinity of the transport system likely facilitates utilization of the low urea concentrations typically found in marine environments.

Key findings

• Deleya venusta possesses an energy-dependent, sodium-dependent urea permease with high affinity (Km = 1.4 µM), distinct from the urease enzyme that hydrolyzes transported urea
• Urea uptake is regulated at two levels: synthesis is subject to nitrogen control (repressed by ammonium), while activity is inhibited by intracellular glutamine rather than extracellular ammonium
• The uptake system depends on electrochemical ion gradients rather than direct ATP hydrolysis, as shown by inhibition with metabolic inhibitors and the protonophore CCCP
• Glutamine synthetase-deficient mutants lose ammonium-dependent inhibition of urea uptake, confirming that glutamine is the regulatory molecule
• The high affinity of the transport system is comparable to marine algae and facilitates urea utilization at the micromolar concentrations found in seawater