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

半胱氨酸的生物合成是植物吸收硫的主要途径。半胱氨酸合成酶（CSase）[O-乙酰丝氨酸（巯基）裂解酶]负责半胱氨酸生物合成的末端步骤，催化O-乙酰-L-丝氨酸（OAS）和硫化氢形成L-半胱氨酸。OAS由来自乙酰辅酶A和丝氨酸的丝氨酸乙酰转移酶（SATase）提供。在植物细胞中至少存在三种CSase亚型，分别定位于细胞质、叶绿体和线粒体中。这些异构体的亚细胞定位通过使用表达转运肽-gus融合基因的转基因植物的体内实验来证实。获得了整合有35 S-CSaseA cDNA、35 S-pearbcS转运肽-CSaseA cDNA和35 S-antisense CSaseA cDNA的转基因烟草植株，并分析了其半胱氨酸生物合成响应于不同硫胁迫的调节。结果表明，外源半胱氨酸合酶在叶绿体中的过量积累可促进半胱氨酸的生物合成。

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.