Molecular Biological Studies on the Regulation of MeEthionine Biosynthesis Using an Arabidopsis thaliana Mutant.

为了了解高等植物甲硫氨酸生物合成调控的分子机制，对过量积累可溶性蛋氨酸的拟南芥突变体mto1进行了研究。Mtol突变体植株在幼嫩的莲座丛叶片中积累了高达40倍的可溶性蛋氨酸，而天冬氨酸家族其他氨基酸的积累与野生型植株没有明显的不同。因此，mtol突变体携带的生化损伤可能存在于该途径的后期步骤之一。胱硫醚-γ-合成酶(CGS)是蛋氨酸生物合成的关键酶，与野生型植物相比，mtol突变体中CGS的mRNA积累较高。当野生型植株在蛋氨酸存在的情况下培养时，CGS mRNA的积累减少，而在mtol突变植株中没有观察到这种影响。因此，推测野生型植物在蛋氨酸过量条件下具有下调CGS基因表达的机制。遗传作图和序列分析表明，mtol突变体在CGS基因编码区存在突变。瞬时表达分析表明，mtol突变影响CGS基因的稳定性。

In order to know the molecular mechanism of the regulation of methionine biosynthesis in higher plants, Arabidopsis mutants, mtol, that over-accumulate soluble methionine was studied. The mtol mutant plants accumulate up to 40-fold higher concentrations of soluble methionine in young rosette leaves, whereas the accumulation of other amino acids of the aspartate family were not appreciably different from wild-type plants. Therefore, the biochemical lesion carried by the mtol mutant was suggested to be resided in one of the later steps in the pathway. Accumulation of mRNA for cystathionine gamma-synthase (CGS), which has been suggested to be a key enzyme in methionine biosynthesis, was higher in mtol mutant as compared to wild-type plants. When wild0type plants were cultured in the presence of methionine, the accumulation of CGS mRNA was reduced, whereas no such effect was observed in the mtol mutant plants. Therefore it was suggested that wild-type plants have a mechanism to down-regulate the accumulation of CGS mRNA in response to methionine excess conditions. Genetic mapping and sequence analyses revealed that the mtol mutant bears mutations in the coding region of CGS gene. Transient expression analyses indicated that the mtol mutation affects the stability of CGS mRNA.