Studies on Molecular Mechanism of Abscisic Acid-Regulated Transcription

水稻碱性区域亮氨酸拉链（bZIP）因子TRAB 1与脱落酸反应元件（ABRE）结合并介导脱落酸（阿坝）信号以激活转录。我们在这里表明，TRAB 1是迅速磷酸化的体内标记实验和磷酸酶敏感的迁移率的SDS-聚丙烯酰胺凝胶上的位移。我们以前已经表明，嵌合启动子含有GAL 4结合位点成为ABA诱导的GAL 4结合域：：TRAB 1融合蛋白表达时。该表达系统使我们能够测定TRAB 1的ABA应答功能。使用该测定系统，我们已经确定TRAB 1的Ser-44对于该功能至关重要。由于ABA诱导的迁移率变化时，没有观察到丝氨酸-44突变为丙氨酸，这丝氨酸残基被认为是磷酸化响应阿坝。细胞分级实验以及荧光显微镜观察瞬时表达的GFP：：TRAB 1融合蛋白表明TRAB 1独立于阿坝定位于细胞核中。我们的研究结果表明，主要阿坝信号转导途径的终点或近终点事件是预先存在的TRAB 1和可能的其他同源ABRE结合因子在细胞核中的磷酸化。

The rice basic region-leucine zipper (bZIP) factor TRAB1 binds to abscisic acid response elements (ABREs) and mediates abscisic acid (ABA) signals to activate transcription. We show here that TRAB1is rapidly phosphorylated by in vivo labeling experiments and by phosphatase-sensitive mobilty-shifts on a SDS-polyacrylamide gel. We previously have shown that a chimeric promoter containing GAL4-binding sites becomes ABA-inducible when a GAL4-binding domain : : TRAB1 fusion protein is expressed. This expression system allowed us to assay the ABA-response function of TRAB1. Using this assay system, we have identified that Ser-44 of TRAB1 is critical for this function. Since ABA-induced mobility-shift was not observed when Ser-44 was mutated to alanine, this serine residue is suggested to be phosphorylated in response to ABA. Cell-fractionation experiments as well as the fluorescent microscopic observation of transiently expressed GFP : : TRAB1 fusion protein indicated that TRAB1 is localized in the nucleus independently of ABA. Our results suggest that the end or near-end point event of the primary ABA signal transduction pathway is the phosphorylation in the nucleus of pre-existing TRAB1 and probablly other homologous ABRE -binding factors.