Sangwan I, O'Brian M R
Department of Biochemistry, State University of New York at Buffalo 14214.
Plant Physiol. 1993 Jul;102(3):829-34. doi: 10.1104/pp.102.3.829.
Extracts of soybean (Glycine max) root nodules and greening etiolated leaves catalyzed radiolabeled delta-aminolevulinic acid (ALA) formation from 3,4-[3H]glutamate but not from 1-[14C]glutamate. Nevertheless, those tissue extracts expressed the activity of glutamate 1-semialdehyde (GSA) aminotransferase, the C5 pathway enzyme that catalyzes ALA synthesis from GSA for tetrapyrrole formation. A soybean nodule cDNA clone that conferred ALA prototrophy, GSA aminotransferase activity, and glutamate-dependent ALA formation activity on an Escherichia coli GSA aminotransferase mutant was isolated. The deduced product of the nodule cDNA shared 79% identity with the GSA aminotransferase expressed in barley leaves, providing, along with the complementation data, strong evidence that the cDNA encodes GSA aminotransferase. GSA aminotransferase mRNA and enzyme activity were expressed in nodules but not in uninfected roots, indicating that the Gsa gene is induced in the symbiotic tissue. The Gsa gene was strongly expressed in leaves of etiolated plantlets independently of light treatment and, to a much lesser extent, in leaves of mature plants. We conclude that GSA aminotransferase, and possibly the C5 pathway, is expressed in a nonphotosynthetic plant organ for nodule heme synthesis and that Gsa is a regulated gene in soybean.
大豆（Glycine max）根瘤提取物以及黄化叶片绿化过程中的提取物可催化从3,4-[³H]谷氨酸而非1-[¹⁴C]谷氨酸形成放射性标记的δ-氨基乙酰丙酸（ALA）。然而，这些组织提取物表现出谷氨酸1-半醛（GSA）氨基转移酶的活性，该酶是C5途径中的一种酶，可催化由GSA合成ALA以形成四吡咯。分离出一个大豆根瘤cDNA克隆，该克隆赋予大肠杆菌GSA氨基转移酶突变体ALA原养型、GSA氨基转移酶活性以及依赖谷氨酸的ALA形成活性。根瘤cDNA推导的产物与大麦叶片中表达的GSA氨基转移酶有79%的同源性，结合互补数据，有力地证明该cDNA编码GSA氨基转移酶。GSA氨基转移酶mRNA和酶活性在根瘤中表达，但在未感染的根中不表达，这表明Gsa基因在共生组织中被诱导。Gsa基因在黄化幼苗的叶片中强烈表达，与光照处理无关，而在成熟植株的叶片中表达程度要低得多。我们得出结论，GSA氨基转移酶，可能还有C5途径，在非光合植物器官中表达以用于根瘤血红素合成，并且Gsa是大豆中的一个受调控基因。
