Localization of nitrogen-related enzymes in higher plants

nadh依赖性谷氨酸合成酶（GOGAT）是高等植物氮素同化和利用的关键酶。然而，对该酶的细胞内定位和分子特性知之甚少。在这项联合研究中，利用免疫金电镜方法证实了该酶蛋白在水稻根系中的细胞内定位。首先，我们确定了NADH-GOGAT蛋白位于根的表皮、外表皮和中央柱体的质体中。我们还成功地从大麦根中纯化了酶。此外，对水稻NADH-GOGAT的基因组克隆和cDNA克隆进行了分离和测序，并对其基因结构进行了深入分析。核苷酸序列已登记在基因库中。研究了该基因在水稻幼苗根系中对氮供应的表达。NADH-GOGAT基因与谷氨酰胺合成酶基因在NH_4^+供给后3小时内特异性诱导。比较粳稻、籼稻和爪哇稻三种水稻发育叶片中NADH-GOGAT蛋白含量，籼稻叶片中NADH-GOGAT蛋白含量低于粳稻叶片。从这些研究中，NADH-GOGAT的分子和生理特性越来越清楚，至少在水稻植物中是这样。

NADH-dependent glutamate synthase (GOGAT) is a key enzyme in nitrogen assimilation and utilization in higher plants. However, little is known on the intracellular localization and molecular properties of this enzyme.In this joint research, intracellular localization of this enzyme protein has been demonstrated by the methods of immuno-gold electron microscopy in rice roots. As the first instance, we determine that the NADH-GOGAT protein locates in plastids of epidermis, exodermis, and central cylinder of the roots. We also succeeded to purify the enzyme from barley roots.Further, genomic clones, as well as cDNA clones, for rice NADH-GOGAT have been isolated and sequenced and the gene structure was thoroughly analyzed. The nucleotide sequences were registered into a Gene-Bank. Expression of this gene has been studied in response to the nitrogen supply in the roots of rice seedlings. The NADH-GOGAT gene is specifically induced within 3 hr after the supply of NH_4^+ among another GOGAT gene and glutamine synthetase genes. When the content of NADH-GOGAT protein in developing leaves were compared among various types of rice, i.e.japonica, indica, and javanica, the leaves in indica-type rice contained less amounts of the protein than those in japonica-type.From those studies, molecular and physiological properties of NADH-GOGAT are getting clear, at least in rice plants.