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 IG...

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，其排布可分为两种类型：独立排布（S型排列，S-type arrangement），或是与其他重复基因连锁排布，多数情况下与编码18S-5.8S-26S rRNA基因的核糖体DNA（rDNA）位点连锁（L型排列，L-type arrangement）。本研究采用Southern印迹杂交、聚合酶链式反应(PCR)及测序技术，对松柏目、银杏目、买麻藤目、苏铁目所有主要裸子植物类群的核糖体RNA（rRNA）基因基因组组织形式进行分析。本研究共收录27个物种（隶属于21个属）的相关数据。与35S核糖体DNA（35S rDNA）单元连锁的5S单元，存在于部分（而非全部）买麻藤目、松柏目物种以及银杏中（约占分析物种的30%），其余物种的5S单元则呈独立排布形式。这类连锁的5S rRNA基因既可作为单拷贝插入序列存在，也可作为短串联单元嵌入26S-18S rDNA基因间区(IGS)中。5S转录本的编码DNA链可与18S-5.8S-26S rRNA基因的编码链相同（如银杏、麻黄属（Ephedra）），或是相反（如罗汉松属（Podocarpus））。此外，在部分IG...中还发现了假基因化的5S拷贝。