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-l1, 76-s) of Rhizobium japonicum strains 3I1b110 and 61a76. Specific activities of the key ammonia assimilatory enzymes, glutamine synthetase (EC 6.3.1.2), glutamate synthase (EC 2.6.1.53) and glutamate dehydrogenase (EC 1.4.1.2), were determined in aerobic, microaerophilic and bacteroid cell-free extracts. In aerobic culture, 110-l1 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-l1 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 R. japonicum derivatives that fix greater amounts of nitrogen also assimilate less fixed nitrogen.
BergersenI. J., TurnerG. L.1967; Nitrogen fixation by the bacteroid fraction of breis of soybean root nodules. Biochimica et biophysica acta 141:507–515
BishopP. E., GuevaraJ. G., EngelkeJ. A., EvansH. J.1976; Relation between glutamine synthetase and nitrogenase activities in the symbiotic association between Rhizobium japonicum and Glycine max. Plant Physiology 57:542–546
KurzW. G. W., RokoshD. A., La RueT. A.1975; Enzymes of ammonia assimilation in Rhizobium leguminosarum bacteroids. Canadian Journal of Microbiology 21:1009–1012
KuykendallL. D., ElkanG. H.1976; Rhizobium japonicum derivatives differing in nitrogen fixing efficiency and carbohydrate utilization. Applied and Environmental Microbiology 32:511–519
LillichT. T., ElkanG. H.1968; Evidence countering the role of polygalacturonase in invasion of root hairs of leguminous plants by Rhizobium spp. Canadian Journal of Microbiology 14:617–625
PrusinerS., MillerR. E., ValentineR. C.1972; Adenosine 3′:5′-cyclic monophosphate control of the enzymes of glutamine metabolism in Escherichia coli. Proceedings of the National Academy of Sciences of the United States of America692922–2926
TjepkemaJ., EvansH. J.1975; Nitrogen fixation by free-living Rhizobium in a defined liquid medium. Biochemical and Biophysical Research Communications 65:625–628
UpchurchR. G., ElkanG. H.1977; Comparison of colony morphology, salt tolerance and effectiveness in Rhizobium japonicum. Canadian Journal of Microbiology 23:1118–1122