Effects of L-methionine-DL-sulphoximine on the assimilation of newly fixed NH3, acetylene reduction and heterocyst production in Anabaena cylindrica. |
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Authors: | W D Stewart P Rowell |
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Institution: | Department of Biological Sciences, University of Dundee, Dundee DD1 4HN, Scotland, U.K. |
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Abstract: | The addition of exogenous L-methionine-DL-sulphoximine (MSO) to N2-fixing cultures of the blue-green alga Anabaena cylindrica results in over half of the newly fixed NH3 being released into the medium. MSO also inhibits glutamine synthetase (GS) activity, has negligible effect on alanine dehydrogenase activity, and glutamate dehydrogenase activity under N2-fixing conditions is negligible. In the presence of MSO, intracellular pools of glutamate and glutamine decrease, those of aspartate and alanine + glycine show little change, and the NH3 pool increases. MSO alleviates the inhibitory effect of exogenous NH4+ on nitrogenase synthesis and heterocyst production. The results suggest that in N2-fixing cultures of A. cylindrica the primary NH3 assimilating pathway involves GS, and probably glutamate synthase (GOGAT), and that the repressor of nitrogenase synthesis and heterocyst production is not NH4+ but is GS, GOGAT, or a product of their reactions. |
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