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Title: Interactions between nitrogen fixation and alternative sources of nitrogen in Gloeothece
Author: Cheng, J.
Awarding Body: University College of Swansea
Current Institution: Swansea University
Date of Award: 1995
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When grown under constant illumination, Gloeothece ATCC 27152, a unicellular cyanobacterium, can use nitrate, nitrite, dinitrogen or ammonium as the sole N-source for growth. The uptake systems for nitrate and nitrite were fully active and ammonium-repressible in N2-fixing cultures. Nitrite uptake was mediated via two pH-dependent systems: passive diffusion of HNO2 and active transport of nitrite. Nitrate uptake was highly light-dependent. Ammonium was also transported by passive diffusion of ammonia and active uptake of ammonium, depending on the pH of the medium. Ammonium inhibited nitrate uptake almost completely, but inhibited nitrite uptake only partially. The true inhibitor was a product of ammonium assimilation, possibly glutamine, rather than ammonium itself, since L-methionine-DL-sulphoximine, an inhibitor of glutamine synthetase blocked GS activity very quickly and decreased the inhibitory effect of ammonium. On the other hand, the inhibitory effect of nitrite on ammonium uptake was stronger than that of nitrate. Nitrate and nitrite competitively inhibited each other's assimilation, occurring at the uptake stage. This suggested that nitrate and nitrite were transported by a common transporter in Gloeothece. The rates of nitrate and nitrite uptake were similar, but the uptake of ammonium was much faster than that of either nitrate of nitrite. In contrast to the uptake systems, systems of nitrate and nitrite reductions were substrate-inducible. Ammonium, either generated intracellularly or supplied exogenously, was assimilated via the GS-GOGAT pathway. Nitrite and ammonium inhibited N2 fixation rapidly. Nitrate inhibited N2 fixation less rapidly and less extensively, and often temporarily stimulated nitrogenase activity. The inhibitory effects of nitrate and ammonium could be prevented by L-methionine-DL-sulphoximine, suggesting that the true inhibitor of N2 fixation was an assimilatory product of ammonium rather than ammonium or nitrate itself.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available