Kobayashi Yuki, Dokiya Yuko, Kumazawa Yoshinori, Sugita Mamoru
Center for Gene Research, Nagoya University, Chikusa, 464-8602, Nagoya, Japan.
Biochem Biophys Res Commun. 2002 Nov 22;299(1):57-61. doi: 10.1016/s0006-291x(02)02579-2.
A third nuclear gene encoding a bacteriophage T7-type RNA polymerase, NsRpoT-C, was isolated and characterized from Nicotiana sylvestris. The gene, NsRpoT-C, consists of 21 exons and 20 introns and encodes a polypeptide of 977 amino acid residues. The predicted NsRpoT-C protein shows the highest identity (72% amino acid identity) with Arabidopsis thaliana RpoT;3 which is a plastid-targeted protein. Surprisingly, comparison of the deduced amino acid sequence of NsRpoT-C with that of A. thaliana RpoT;3 predicted that the NsRpoT-C starts at a CUG triplet, a rare translation initiation codon. Transient expression assays in protoplasts from tobacco leaves demonstrated that the putative N-terminal transit peptide of NsRpoT-C encodes a targeting signal directing the protein into chloroplasts. This strongly suggests that NsRpoT-C functions as an RNA polymerase transcribing plastid-encoded genes. We have designated this protein NsRpoTp.
从野生烟草中分离并鉴定了第三个编码噬菌体T7型RNA聚合酶的核基因NsRpoT-C。该基因NsRpoT-C由21个外显子和20个内含子组成，编码一个含有977个氨基酸残基的多肽。预测的NsRpoT-C蛋白与拟南芥RpoT;3具有最高的同一性（72%氨基酸同一性），RpoT;3是一种定位于质体的蛋白。令人惊讶的是，将NsRpoT-C推导的氨基酸序列与拟南芥RpoT;3的氨基酸序列进行比较预测，NsRpoT-C起始于CUG三联体，这是一个罕见的翻译起始密码子。烟草叶片原生质体中的瞬时表达分析表明，NsRpoT-C假定的N端转运肽编码一个将蛋白质导向叶绿体的靶向信号。这有力地表明NsRpoT-C作为一种转录质体编码基因的RNA聚合酶发挥作用。我们将这种蛋白命名为NsRpoTp。
