Sangwan I, O'Brian M R
Department of Biochemistry, State University of New York, Buffalo, New York 14214, USA.
Plant Physiol. 1999 Feb;119(2):593-8. doi: 10.1104/pp.119.2.593.
Heme and chlorophyll accumulate to high levels in legume root nodules and in photosynthetic tissues, respectively, and they are both derived from the universal tetrapyrrole precursor delta-aminolevulinic acid (ALA). The first committed step in ALA and tetrapyrrole synthesis is catalyzed by glutamyl-tRNA reductase (GTR) in plants. A soybean (Glycine max) root-nodule cDNA encoding GTR was isolated by complementation of an Escherichia coli GTR-defective mutant for restoration of ALA prototrophy. Gtr mRNA was very low in uninfected roots but accumulated to high levels in root nodules. The induction of Gtr mRNA in developing nodules was subsequent to that of the gene Enod2 (early nodule) and coincided with leghemoglobin mRNA accumulation. Genomic analysis revealed two Gtr genes, Gtr1 and a 3' portion of Gtr2, which were isolated from the soybean genome. RNase-protection analysis using probes specific to Gtr1 and Gtr2 showed that both genes were expressed, but Gtr1 mRNA accumulated to significantly higher levels. In addition, the qualitative patterns of expression of Gtr1 and Gtr2 were similar to each other and to total Gtr mRNA in leaves and nodules of mature plants and etiolated plantlets. The data indicate that Gtr1 is universal for tetrapyrrole synthesis and that a Gtr gene specific for a tissue or tetrapyrrole is unlikely. We suggest that ALA synthesis in specialized root nodules involves an altered spatial expression of genes that are otherwise induced strongly only in photosynthetic tissues of uninfected plants.
血红素和叶绿素分别在豆科植物根瘤和光合组织中大量积累，它们都源自通用的四吡咯前体δ-氨基乙酰丙酸（ALA）。植物中ALA和四吡咯合成的首个关键步骤由谷氨酰-tRNA还原酶（GTR）催化。通过互补大肠杆菌GTR缺陷型突变体以恢复ALA原养型，分离得到了一个编码大豆（Glycine max）根瘤GTR的cDNA。Gtr mRNA在未感染的根中含量极低，但在根瘤中大量积累。在发育中的根瘤中，Gtr mRNA的诱导发生在Enod2（早期根瘤）基因之后，与豆血红蛋白mRNA的积累同时发生。基因组分析揭示了两个Gtr基因，Gtr1和Gtr2的3'部分，它们是从大豆基因组中分离出来的。使用针对Gtr1和Gtr2的特异性探针进行的核糖核酸酶保护分析表明，这两个基因均有表达，但Gtr1 mRNA的积累水平显著更高。此外，成熟植株和黄化幼苗的叶片及根瘤中，Gtr1和Gtr2的定性表达模式彼此相似，且与总Gtr mRNA的表达模式相似。数据表明，Gtr1对于四吡咯合成具有普遍性，不太可能存在针对特定组织或四吡咯的Gtr基因。我们认为，在特化的根瘤中，ALA合成涉及基因空间表达的改变，而这些基因在未感染植物的光合组织中通常仅被强烈诱导。
