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Recent work has shown that glutamine synthetase (EC 6.3.1.2), the key enzyme of ammonia metabolism, occurs in multiple molecular forms in many higher plants (1, 7, 12, 15, 16, 18-20, 23, 27). Ion exchange chromatography and subcellular localization studies have shown the presence of two isoenzymes in photosynthetically active tissue: GS, is localized in the cytoplasm whereas GS2 is found in chloroplasts (10,11,17). These isoenzymes differ in several respects, including their pH optima, heat stability, and Km for glutamate (1,9,16,18,19 Extracts from cells of Chlorella grown under a range of nutritional conditions were found to exhibit two peaks of glutamine synthetase activity when subjected to ion exchange chromatography (Fig. 1). The first peak eluted at 70 mm KCI and the second at 230 mm KCI, and by analogy with the corresponding chromatographic forms in higher plants they are designated GS, and GS2 (see Refs. 9 and 16). GS, was always the major form present, comprising 60 to 80% of the total activity. It is evident from the results in Figure 1 and those summarized in Table I

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Cytosolic glutamine synthetase (GS,) was purified to homogeneity from etiolated barley leaves by DEAESephacel and hydroxyapatite chromatography, gel filtration and polyacrylamide gel electrophoresis. Specific antibodies against the purified protein were raised by the immunization of rabbits. Immunoprecipitation experiments demonstrated that cytosolic glutamine synthetases isolated from the leaves of different plant species were very similar proteins. Good recognition of other cytosolic glutamine synthetases from roots, root nodular tissue and seeds by barley GS, antibodies was obtained, suggesting that they too are all quite similar proteins. In contrast, chloroplast glutamine synthetase (GS,) was considered to be a different protein in view of its low level of recognition by barley GS, antibodies.