Department of Bacteriology, University of California, 95616, Davis, CA, USA.
Planta. 1985 Jun;164(3):406-14. doi: 10.1007/BF00402954.
The initial product of fixation of [(13)N]N2 by pure cultures of the reconstituted symbiotic association between Anthoceros punctatus L. and Nostoc sp. strain ac 7801 was ammonium; it accounted for 75% of the total radioactivity recovered in methanolic extracts after 0.5 min and 14% after 10 min of incubation. Glutamine and glutamate were the primary organic products synthesized from [(13)N]N2 after incubation times of 0.5-10 min. The kinetics of labeling of these two amino acids were characteristic of a precursor (glutamine) and product (glutamate) relationship. Results of inhibition experiments with methionine sulfoximine (MSX) and diazo-oxonorleucine were also consistent with the assimilation of N2-derived NH 4 (+) by Anthoceros-Nostoc through the sequential activities of glutamine synthetase (EC 6.3.1.2) and glutamate synthase (EC 1.4.7.1), with little or no assimilation by glutamate dehydrogenase (EC 1.3.1.3). Isolated symbiotic Nostoc assimilated exogenous (13)NH 4 (+) into glutamine and glutamate and their formation was inhibited by MSX, indicating operation of the glutamine synthetase-glutamate synthase (GS-GOGAT) pathway: However, relative to free-living cultures, isolated symbiotic Nostoc assimilated 80% less exogenous ammonium into glutamine and glutamate, implying that symbiotic Nostoc could assimilate only a fraction of N2-derived NH 4 (+) . This implication was tested by using Anthoceros associations reconstituted with wild-type or MSX-resistant strains of Nostoc incubated with [(13)N]N2 in the presence of MSX. The results of these experiments indicated that, in situ, symbiotic Nostoc assimilated about 10% of the N2-derived NH 4 (+) and that NH 4 (+) was made available to Anthoceros tissue where it was apparently assimilated by the GS-GOGAT pathway. Since less than 1% of the fixed N2 was lost to the suspension medium, it appears that transfer of NH 4 (+) from symbiont to host tissue was very efficient in this extracellular symbiotic association.
纯培养的角星蕨（Anthoceros punctatus L.）和念珠藻（ Nostoc sp. strain ac 7801）共生体可将 [(13)N]N2 固定为初始产物铵；在孵育 0.5 分钟后，甲醇提取物中回收的总放射性的 75%为铵，孵育 10 分钟后，有 14%为铵。在 0.5-10 分钟的孵育时间后， [(13)N]N2 合成的主要有机产物是谷氨酰胺和谷氨酸。这两种氨基酸的标记动力学特征是前体（谷氨酰胺）和产物（谷氨酸）关系。用甲硫氨酸亚砜（MSX）和重氮氧代正亮氨酸进行抑制实验的结果也与角星蕨-念珠藻通过谷氨酰胺合成酶（EC 6.3.1.2）和谷氨酸合酶（EC 1.4.7.1）的顺序活性同化 N2 衍生的 NH 4 (+)一致，谷氨酸脱氢酶（EC 1.3.1.3）同化很少或没有同化。分离共生的念珠藻将外源性 (13)NH 4 (+) 同化到谷氨酰胺和谷氨酸中，MSX 抑制其形成，表明谷氨酰胺合成酶-谷氨酸合酶（GS-GOGAT）途径的运转：然而，与自由生活的培养物相比，分离共生的念珠藻将 80% 更少的外源性铵同化到谷氨酰胺和谷氨酸中，这意味着共生的念珠藻只能同化一部分 N2 衍生的 NH 4 (+)。通过使用含有 [(13)N]N2 的角星蕨与野生型或 MSX 抗性念珠藻的重建共生体来测试这种含义，并在 MSX 存在下孵育。这些实验的结果表明，在原位，共生的念珠藻同化了大约 10% 的 N2 衍生的 NH 4 (+)，并且 NH 4 (+) 可用于角星蕨组织，其中显然通过 GS-GOGAT 途径进行同化。由于固定的 N2 中只有不到 1% 损失到悬浮培养基中，因此在这种细胞外共生体中，从共生体到宿主组织的 NH 4 (+) 转移效率非常高。
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