Hedtke Boris, Legen Julia, Weihe Andreas, Herrmann Reinhold G, Börner Thomas
Institut für Biologie, Humboldt-Universität, Chausseestr. 117, D-10115 Berlin, Germany.
Plant J. 2002 Jun;30(6):625-37. doi: 10.1046/j.1365-313x.2002.01318.x.
In higher plants, a small nuclear gene family encodes mitochondrial as well as chloroplast RNA polymerases (RNAP) homologous to the bacteriophage T7-enzyme. The Arabidopsis genome contains three such RpoT genes, while in monocotyledonous plants only two copies have been found. Analysis of Nicotiana tabacum, a natural allotetraploid, identified six different RpoT sequences. The study of the progenitor species of tobacco, N. sylvestris and N. tomentosiformis, uncovered that the sequences represent two orthologous sets each of three RpoT genes (RpoT1, RpoT2 and RpoT3). Interestingly, while the organelles are inherited exclusively from the N. sylvestris maternal parent, all six RpoT genes are expressed in N. tabacum. GFP-fusions of Nicotiana RpoT1 revealed mitochondrial targeting properties. Constructs containing the amino-terminus of RpoT2 were imported into mitochondria as well as into plastids. Thus, the dual-targeting feature, first described for Arabidopsis RpoT;2, appears to be conserved among eudicotyledonous plants. Tobacco RpoT3 is targeted to chloroplasts and the RNA is differentially expressed in plants lacking the plastid-encoded RNAP. Remarkably, translation of RpoT3 mRNA has to be initiated at a CUG codon to generate a functional plastid transit peptide. Thus, besides AGAMOUS in Arabidopsis, Nicotiana RpoT3 provides a second example for a non-viral plant mRNA that is exclusively translated from a non-AUG codon.
在高等植物中，一个小的核基因家族编码与噬菌体T7酶同源的线粒体和叶绿体RNA聚合酶（RNAP）。拟南芥基因组包含三个这样的RpoT基因，而在单子叶植物中仅发现了两个拷贝。对天然异源四倍体烟草的分析鉴定出六个不同的RpoT序列。对烟草的祖先物种野生烟草和绒毛状烟草的研究发现，这些序列代表两个直系同源组，每组各有三个RpoT基因（RpoT1、RpoT2和RpoT3）。有趣的是，虽然细胞器仅从野生烟草母本遗传，但所有六个RpoT基因都在烟草中表达。烟草RpoT1的绿色荧光蛋白融合物显示出线粒体靶向特性。含有RpoT2氨基末端的构建体被导入线粒体以及质体。因此，首次在拟南芥RpoT;2中描述的双靶向特征似乎在双子叶植物中是保守的。烟草RpoT3靶向叶绿体，并且该RNA在缺乏质体编码的RNAP的植物中差异表达。值得注意的是，RpoT3 mRNA的翻译必须在CUG密码子处起始以产生功能性的质体转运肽。因此，除了拟南芥中的AGAMOUS之外，烟草RpoT3为仅从非AUG密码子翻译的非病毒植物mRNA提供了第二个例子。
