香气是茶叶的重要品质之一,萜类物质不仅香气好,而且沸点普遍较高,是构成茶叶香气的重要物质基础,决定着茶叶的香气品质,也可作为茶叶香型划分的依据.在植物中,倍半萜、多萜醇等通过胞质中的甲瓦龙酸(MVA)途径合成.HMG-CoA 还原酶(HMGR)催化 HMG-CoA(3-羟基-3-甲基戊二酸单酰辅酶 A)生成甲瓦龙酸,是依赖 MVA 萜类合成途径的关键限速反应.为了有助于理解茶树萜类合成的分子遗传机制,通过 RACE-PCR 方法从茶树中克隆了一个编码 HMG-CoA 还原酶的 cDNA 全长序列(命名为 CsHMGR 1),该序列由1979 bp 组成,包含一个1722 bp 的完整开放阅读框,编码573个氨基酸.其推定的编码蛋白与橡胶树、旱莲木、人参、荔枝、西洋参、丹参、罗汉果及龙眼的同源蛋白具有80％~82％的序列一致性.利用 CsH-MGR1和其它物种 HMGR 同源蛋白的催化区域构建系统发育树,表明其属于真核生物 I 类 HMGR 家族.结构分析表明,CsHMGR1含有两个跨膜区,推测其与其它真核生物同源蛋白类似地定位于内质网上；含有两个HMG-CoA 结合位点、两个 NADPH 结合位点、四个保守的催化活性残基及一个磷酸化位点,说明磷酸化/去磷酸化很可能也是其活性调节的重要方式.表达分析表明,CsHMGR 1在“大叶龙”叶芽、母株叶芽及花芽都有较强的表达.其表达调控及生理活性对茶叶品质可能有重要影响,并在其功能解析的基础上,有可能作为茶叶品质鉴定及育种的一个依据.%Aroma is an important character of tea,and terpenoids,with preferable fragrance and relatively high boiling point,are substantial substances contributing to tea aroma,which not only determine the aroma quality of tea,but also provide a classification basis of scent types.In plants,sesquiterpenes and dolichol and so forth are synthesize via the cytoplasmic mevalonate-dependent pathway.The enzyme HMG-CoA reductase (HMGR)catalyzes the conversion of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA)to mevalonate,which is the committed step in the synthesis of isoprenoids via the MVA pathway.To help understand the molecular and genetic mechanisms underlying terpenoid synthesis of tea,a full-length cDNA encoding HMGR was cloned from tea (Camellia sinensis (L.)O.Kuntze)by using the RACE-PCR technique (designated as CsHMGR 1).It comprised 1 979 bp,with a 1 722 bp intact open read-ing frame encoding a 573-amino-acid protein.The deduced protein showed 80% to 82% similarities to homologs from rubber tree (Hevea brasiliensis ),common camptotheca fruit (Camptotheca acuminate ),ginseng (Panax ginseng ), litchis (Litchi chinensis ),American ginseng (Panax quinquefolius ),rooted salvia (Salvia miltiorrhiza ),Momordica grosvenori (Siraltia grosvenorii ),and longan (Dimocarpus longan).The phylogenetic tree,constructed with the cat-alytic domalned of CsHMGR1 and homologs from other species,indicated that CsHMGR1 belonged to the eukaryotic class I HMGR family.CsHMGR1 consisted of two transmembrane domalns,implying that it may be localized to en-doplasmic reticulum (ER)similarly to other eukaryotic homologs.It also contalned two HMG-CoA binding sites,two NADP(H)-binding sites,four conserved catalytic active residues and a phosphorylation site,indicating that phospho-rylation/dephosphorylation is likely a crucial mode of regulation of its biochemical activity.Tissue expression analysis indicated that CsHMGR 1 was expressed comparatively in the leaf buds of C .sinensis cv.Dayelong and in both leaf buds and floral buds of the mother plants.The regulation of expression and physiological activity of CsHMGR1 are likely to impact greatly on tea quality,and CsHMGR1 may provide a basis of the quality evaluation and breeding of tea given that its function is further resolved.
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