Molecular mechanism of regulation of methionine biosynthesis by mRNA stability : Studies using mto1 mutants of Arabidopsis.

拟南芥胱胱甘氨酸-合成酶（CGS）含有563个氨基酸，其第一个外显子编码183个氨基酸。比较高等植物的CGS序列，发现外显子1内有一个35个氨基酸的区域（“MTO1区”），该区域在植物中高度保守。提示该区域在CGS基因表达调控中具有一定的作用。CGS外显子1内的序列缺失突变体由n和c端构建。将缺失构建体与报告基因-葡萄糖醛酸酶（GUS）融合，置于花椰菜花叶病毒（CaMV） 35S RNA启动子的控制下，通过瞬时表达实验分析其对Met应用的反应。GUS实验结果表明，携带MTO1区域的构建体对Met有响应，而不携带MTO1区域的构建体对Met没有响应。此外，当将MTO1 -1突变引入那些携带MTO1区域的结构体时，应答消失。还构建了MTO1两侧的缺失，并测试了它们对Met应用的响应。结果表明，MTO1区域足以响应Met的下调。

Cystathionine gamma-synthase (CGS) of Arabidopsis has 563 amino acids and the first exon codes for 183 amino acids. Comparison of CGS sequence among higher plants reveals a 35 amino acid region ("MTO1 region") within the exon 1 that is highly conserved among plant species. This suggests that this region has a role in the regulation of CGS gene expression.Sequential deletion mutants within the CGS exon 1 was constructed from both the N-and C-termini. The deletion constructs were fused in-frame with a reporter gene, beta-glucuronidase (GUS), and placed under the control of cauliflower mosaic virus (CaMV) 35S RNA promoter and their response to Met application was analyzed by transient expression experiments. The results of GUS assay indicated that the constructs carrying the MTO1 region respond to Met application while those do not carry the MTO1 region do not. In addition, when mto1-1 mutation was introduced to those constructs that carry the MTO1 region, the response disappeared.Deletions in both sides of MTO1 were also constructed and tested for their response to Met application. The results suggest that the MTO1 region is sufficient for the downregulation in response to Met application.