Transgenic Plants with Over-expressed Cysteine Synthase for Environmental Resistance of Sulfur

半胱氨酸的生物合成是植物硫的主要同化机制。半胱氨酸合成酶（CSase） [o -乙酰丝氨酸（硫醇）裂解酶]负责半胱氨酸生物合成的最后一步，催化o -乙酰丝氨酸（OAS）和硫化氢生成l-半胱氨酸。OAS由乙酰辅酶a和丝氨酸的丝氨酸乙酰转移酶（SATase）提供。csase在植物细胞中至少存在三种亚型，分别定位于细胞质、叶绿体和线粒体。这些亚型的亚细胞定位是通过体内实验用表达转运肽-gus融合基因的转基因植物证实的。利用35S-CSaseA cDNA、35S-pearbcS转运肽-CSaseA cDNA和35s -反义CSaseA cDNA构建转基因烟草植株，分析其在不同硫胁迫下对半胱氨酸生物合成的调节作用。结果表明，外源半胱氨酸合成酶在叶绿体中过量积累可促进半胱氨酸的生物合成流量。

Cysteine biosynthesis is the major assimilation mechanism of sulfur in plants. Cysteine synthase (CSase) [O-acetylserine (thiol) -lyase] responsible for the terminal step of cysteine biosynthesis catalyzes the formation of L-cysteine from O-acetyl-L-serine (OAS) and hydrogen sulfide. OAS is supplied by serine acetyltransferase (SATase) from acetyl-CoA and serine. There are at least three isoforms of CSases in the plant cells, which are localized in cytoplasm, chloroplasts and mitochondria, respectively. The subcellular localization of these isoforms was confirmed by in vivo experiments using transgenic plants expressing the transit peptide-gus fusion genes. Transgenic tobacco plants, integrated with the constructs for 35S-CSaseA cDNA,35S-pearbcS transit peptide-CSaseA cDNA and 35S-antisense CSaseA cDNA,were obtained and analyzed for modulation of cysteine biosynthesis in responese to various sulfur stress. The results indicated that over-accumulated foreign cysteine synthase in chloroplasts could enhance biosynthetic flow of cysteine.