Data from: Dancing together and separate again: gymnosperms exhibit frequent changes of fundamental 5S and 35S rRNA genes (rDNA) organisation

In higher eukaryotes, the 5S rRNA genes occur in tandem units and are arranged either separately (S-type arrangement) or linked to other repeated genes, in most cases to rDNA locus encoding 18S–5.8S–26S genes (L-type arrangement). Here we used Southern blot hybridisation, PCR and sequencing approaches to analyse genomic organisation of rRNA genes in all large gymnosperm groups, including Coniferales, Ginkgoales, Gnetales and Cycadales. The data are provided for 27 species (21 genera). The 5S units linked to the 35S rDNA units occur in some but not all Gnetales, Coniferales and in Ginkgo (~30% of the species analysed), while the remaining exhibit separate organisation. The linked 5S rRNA genes may occur as single-copy insertions or as short tandems embedded in the 26S–18S rDNA intergenic spacer (IGS). The 5S transcript may be encoded by the same (Ginkgo, Ephedra) or opposite (Podocarpus) DNA strand as the 18S–5.8S–26S genes. In addition, pseudogenised 5S copies were also found in some IGS types. Both L- and S-type units have been largely homogenised across the genomes. Phylogenetic relationships based on the comparison of 5S coding sequences suggest that the 5S genes independently inserted IGS at least three times in the course of gymnosperm evolution. Frequent transpositions and rearrangements of basic units indicate relatively relaxed selection pressures imposed on genomic organisation of 5S genes in plants.在高等真核生物中，5S核糖体RNA（5S rRNA）基因以串联单元形式存在，可单独排布（S型排列），或与其他重复基因连锁，在多数情况下与编码18S-5.8S-26S核糖体RNA基因的核糖体DNA（rDNA）位点连锁（L型排列）。本研究采用Southern印迹杂交（Southern blot hybridisation）、聚合酶链式反应（PCR）及测序技术，对松柏目、银杏目、买麻藤目、苏铁目所有大型裸子植物类群的核糖体RNA基因基因组组织形式进行分析。本研究涵盖27个物种（隶属于21个属）的相关数据。与35S rDNA单元连锁的5S单元存在于部分（而非全部）买麻藤目、松柏目及银杏属物种中（约占所分析物种的30%），其余物种的5S基因则呈单独排布形式。连锁的5S rRNA基因可作为单拷贝插入片段，或以嵌入26S-18S rDNA基因间隔区（IGS）的短串联序列形式存在。5S转录本的编码链可与18S-5.8S-26S基因相同（如银杏属、麻黄属），或相反（如罗汉松属）。此外，在部分IGS类型中还发现了假基因化的5S拷贝。L型与S型两种单元在基因组中已基本实现均一化。基于5S编码序列比对构建的系统发育关系表明，在裸子植物演化过程中，5S基因独立插入基因间隔区（IGS）的事件至少发生了三次。基础单元的频繁转座与重排现象说明，植物中5S基因的基因组组织所承受的选择压力相对宽松。