Max-Planck-Institut für Molekulare Pflanzenphysiologie, Am Mühlenberg 1, D-14476 Potsdam-Golm, Germany.
Nucleic Acids Res. 2012 Aug;40(14):6713-24. doi: 10.1093/nar/gks350. Epub 2012 May 2.
The plastid (chloroplast) genomes of seed plants typically encode 30 tRNAs. Employing wobble and superwobble mechanisms, most codon boxes are read by only one or two tRNA species. The reduced set of plastid tRNAs follows the evolutionary trend of organellar genomes to shrink in size and coding capacity. A notable exception is the AUN codon box specifying methionine and isoleucine, which is decoded by four tRNA species in nearly all seed plants. However, three of these four tRNA genes were lost from the genomes of some parasitic plastid-containing lineages, possibly suggesting that less than four tRNA species could be sufficient to decode the triplets in the AUN box. To test this hypothesis, we have performed knockout experiments for the four AUN-decoding tRNAs in tobacco (Nicotiana tabacum) plastids. We find that all four tRNA genes are essential under both autotrophic and heterotrophic growth conditions, possibly suggesting tRNA import into plastids of parasitic plastid-bearing species. Phylogenetic analysis of the four plastid tRNA genes reveals striking conservation of all those bacterial features that are involved in discrimination between the different tRNA species containing CAU anticodons.
植物质体（叶绿体）基因组通常编码 30 种 tRNA。大多数密码子框通过一个或两个 tRNA 物种进行读取，利用摆动和超摆动机制。质体 tRNA 的减少集合遵循细胞器基因组缩小大小和编码能力的进化趋势。一个值得注意的例外是指定甲硫氨酸和异亮氨酸的 AUN 密码子框，在几乎所有种子植物中，由四种 tRNA 物种解码。然而，这四种 tRNA 基因中的三种已从某些寄生质体含有的谱系的基因组中丢失，这可能表明，少于四种 tRNA 物种就足以解码 AUN 框中的三核苷酸。为了检验这一假设，我们在烟草（Nicotiana tabacum）质体中对四种 AUN 解码 tRNA 进行了敲除实验。我们发现，在自养和异养生长条件下，所有四个 tRNA 基因都是必需的，这可能表明质体能够将 tRNA 导入寄生质体含有的物种的质体中。对这四个质体 tRNA 基因的系统发育分析揭示了所有涉及区分含有 CAU 反密码子的不同 tRNA 物种的细菌特征的惊人保守性。
