SUMMARY: Ammonia assimilatory activities were investigated in cultures of small, efficient nitrogen-fixing derivatives (110-I, 76-NS) and large, inefficient nitrogen-fixing derivatives (110-L, 76-s) of strains 3I1b110 and 61A76. Specific activities of the key ammonia assimilatory enzymes, glutamine synthetase (EC, glutamate synthase (EC and glutamate dehydrogenase (EC, were determined in aerobic, microaerophilic and bacteroid cell-free extracts. In aerobic culture, 110-L and 76-s assimilated more ammonia than 110-I and 76-NS because of apparent twofold or greater assimilatory enzyme activities. Specific ammonia assimilatory enzyme activities were 10- to 100-fold lower for all derivatives in nitrogen-fixing microaerophilic and bacteroid cultures. In addition to these already low ammonia assimilatory activities, the assimilatory activities of 110-I and 76-NS were twofold lower than those of 110-L and 76-s. The small colony types also excreted ammonia under nitrogen-fixing conditions.

These findings support the idea that rhizobia can simultaneously derepress nitrogenase biosynthesis whilst repressing ammonia assimilatory enzyme biosynthesis. This investigation has also linked the efficiency of ammonia assimilation with the efficiency of nitrogenase activity as an inverse function, i.e. those derivatives that fix greater amounts of nitrogen also assimilate less fixed nitrogen.


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