Frustaci J M, Sangwan I, O'Brian M R
Department of Biochemistry, State University of New York, Buffalo 14214, USA.
J Biol Chem. 1995 Mar 31;270(13):7387-93. doi: 10.1074/jbc.270.13.7387.
Expression of plant tetrapyrroles is high in photosynthetic tissues and in legume root nodules in the form of chlorophyll and heme, respectively. The universal tetrapyrrole precursor delta-aminolevulinic acid (ALA) is synthesized from glutamate 1-semialdehyde (GSA) by GSA aminotransferase in plants, which is encoded by gsa. Immunoblot analysis showed that GSA aminotransferase was expressed in soybean leaves and nodules, but not in roots, and that protein correlated with enzyme activity. These observations indicate that GSA aminotransferase expression is controlled in tetrapyrrole formation and argue against significant activity of an enzyme other than the well described aminotransferase for GSA-dependent ALA formation. gas mRNA and protein were induced in soybean nodules, and their activation was temporally intermediate between those of the respective early and late genes endo2 and lb. A GSA aminotransferase gene, designated gsa1, was isolated and appears to be one of two gsa genes in the soybean genome. gsa1 mRNA accumulated to high levels in leaves and nodules, but not in uninfected roots as discerned with a gsa1-specific probe. Message levels were higher in leaves from etiolated plantlets than in mature plants, and expression in the former was slightly elevated by light. The expression pattern of gsa1 mRNA was qualitatively similar to that of total gsa. The data strongly suggest that gsa1 is a universal tetrapyrrole synthesis gene and that a gsa gene specific for a tissue, tetrapyrrole, or light condition is unlikely. The gsa1 promoter contained a genetic element found in numerous Drosophila melanogaster genes; the so-called GAGA element displayed single-stranded character in vitro and formed a complex with nuclear factors from nodules and leaves but not from roots. From these observations we infer that the GAGA element is involved in the transcriptional control of gsa1.
植物四吡咯以叶绿素和血红素的形式分别在光合组织和豆科植物根瘤中大量表达。通用的四吡咯前体δ-氨基乙酰丙酸（ALA）在植物中由谷氨酸-1-半醛（GSA）通过GSA转氨酶合成，该酶由gsa编码。免疫印迹分析表明，GSA转氨酶在大豆叶片和根瘤中表达，但在根中不表达，且该蛋白与酶活性相关。这些观察结果表明，GSA转氨酶的表达在四吡咯形成过程中受到调控，并且反对除了已描述的依赖GSA的ALA形成的转氨酶之外的其他酶具有显著活性的观点。gas mRNA和蛋白在大豆根瘤中被诱导，其激活时间介于各自的早期和晚期基因endo2和lb之间。一个名为gsa1的GSA转氨酶基因被分离出来，它似乎是大豆基因组中两个gsa基因之一。用gsa1特异性探针检测发现，gsa1 mRNA在叶片和根瘤中积累到高水平，但在未感染的根中未检测到。黄化幼苗叶片中的信息水平高于成熟植株，并且前者的表达在光照下略有升高。gsa1 mRNA的表达模式在质量上与总gsa相似。数据强烈表明gsa1是一个通用的四吡咯合成基因，并且不太可能存在针对特定组织、四吡咯或光照条件的gsa基因。gsa1启动子包含在许多黑腹果蝇基因中发现的一个遗传元件；所谓的GAGA元件在体外显示出单链特征，并与来自根瘤和叶片而非根的核因子形成复合物。从这些观察结果我们推断，GAGA元件参与了gsa1的转录调控。
