Faculty of Agriculture, Shinshu University, Minamiminowa 8304, Nagano 399-4598, Japan.
Graduate School of Environmental and Life Science, Okayama University, Okayama, Okayama 700-8530, Japan.
Biosci Rep. 2018 Mar 9;38(2). doi: 10.1042/BSR20171457. Print 2018 Apr 27.
Isochorismate synthase (ICS) converts chorismate into isochorismate, a precursor of primary and secondary metabolites including salicylic acid (SA). SA plays important roles in responses to stress conditions in plants. Many studies have suggested that the function of plant ICSs is regulated at the transcriptional level. In , the expression of is induced by stress conditions in parallel with SA synthesis, and is required for SA synthesis. In contrast, the expression of is not induced when SA synthesis is activated in tobacco, and it is unlikely to be involved in SA synthesis. Studies on the biochemical properties of plant ICSs are limited, compared with those on transcriptional regulation. We analyzed the biochemical properties of four plant ICSs: AtICS1, NtICS, NbICS from , and OsICS from rice. Multiple sequence alignment analysis revealed that their primary structures were well conserved, and predicted key residues for ICS activity were almost completely conserved. However, AtICS1 showed much higher activity than the other ICSs when expressed in and leaves. Moreover, the levels of AtICS1 protein expression in leaves were higher than the other ICSs. Construction and analysis of chimeras between AtICS1 and OsICS revealed that the putative chloroplast transit peptides (TPs) significantly affected the levels of protein accumulation in leaves. Chimeric and point-mutation analyses revealed that Thr, Ser, and Ile of AtICS1 are essential for its high activity. These distinct biochemical properties of plant ICSs may suggest different roles in their respective plant species.
分支酸合酶（ICS）将分支酸转化为异分支酸，异分支酸是包括水杨酸（SA）在内的初级和次级代谢物的前体。SA 在植物应对胁迫条件的过程中发挥着重要作用。许多研究表明，植物 ICS 的功能受到转录水平的调控。在 中，与 SA 合成平行诱导 的表达，并且 对于 SA 合成是必需的。相比之下，当烟草中 SA 合成被激活时， 的表达不会被诱导，并且它不太可能参与 SA 合成。与转录调控相比，植物 ICS 的生化特性研究受到限制。我们分析了四种植物 ICS 的生化特性：来自拟南芥的 AtICS1、来自烟草的 NtICS、来自 的 NbICS 和来自水稻的 OsICS。多重序列比对分析表明，它们的一级结构高度保守，并且预测的 ICS 活性关键残基几乎完全保守。然而，当在 和 叶片中表达时，AtICS1 表现出比其他 ICS 更高的活性。此外，在 叶片中 AtICS1 蛋白表达水平高于其他 ICS。AtICS1 和 OsICS 之间嵌合体的构建和分析表明，假定的叶绿体转运肽（TP）显著影响了在 叶片中蛋白质积累的水平。嵌合体和点突变分析表明，AtICS1 的 Thr、Ser 和 Ile 对其高活性是必需的。这些植物 ICS 的不同生化特性可能表明它们在各自的植物物种中发挥着不同的作用。
