The chloroplast genomes of most plants and algae contain a large inverted repeat (IR) region that separates two single-copy regions and harbours the ribosomal RNA operon. We have addressed the functional importance of the IR region by removing an entire copy of the 25.3-kb IR from the tobacco plastid genome. Using plastid transformation and subsequent selectable marker gene elimination, we precisely excised the IR, thus generating plants with a substantially reduced plastid genome size. We show that the lack of the IR results in a mildly reduced plastid ribosome number, suggesting a gene dosage benefit from the duplicated presence of the ribosomal RNA operon. Moreover, the IR deletion plants contain an increased number of plastid genomes, suggesting that genome copy number is regulated by measuring total plastid DNA content rather than by counting genomes. Together, our findings (1) demonstrate that the IR can enhance the translation capacity of the plastid, (2) reveal the relationship between genome size and genome copy number, and (3) provide a simplified plastid genome structure that will facilitate future synthetic biology applications.

大多数植物和藻类的叶绿体基因组包含一个大的反向重复 (IR) 区域，该区域分隔两个单拷贝区域并包含核糖体 RNA 操纵子。我们通过从烟草质体基因组中去除 25.3 kb IR 的整个副本来解决 IR 区域的功能重要性。通过质体转化和随后的选择标记基因消除，我们精确地切除了IR，从而产生了质体基因组大小显着减小的植物。我们发现，缺乏IR会导致质体核糖体数量轻度减少，这表明核糖体RNA操纵子的重复存在可以使基因剂量受益。此外，IR缺失植物含有数量增加的质体基因组，这表明基因组拷贝数是通过测量总质体DNA含量而不是通过基因组计数来调节的。总之，我们的研究结果 (1) 证明 IR 可以增强质体的翻译能力，(2) 揭示基因组大小和基因组拷贝数之间的关系，(3) 提供简化的质体基因组结构，这将有利于未来的合成生物学应用程序。