Department of Medical Biochemistry and Cell Biology, Institute of Biomedicine, Sahlgrenska Academy at University of Gothenburg, Box 440, SE-40530 Göteborg, Sweden.
BMC Genomics. 2010 Feb 11;11:106. doi: 10.1186/1471-2164-11-106.
Two categories of introns are known, a common U2 type and a rare U12 type. These two types of introns are removed by distinct spliceosomes. The phylogenetic distribution of spliceosomal RNAs that are characteristic of the U12 spliceosome, i.e. the U11, U12, U4atac and U6atac RNAs, suggest that U12 spliceosomes were lost in many phylogenetic groups. We have now examined the distribution of U2 and U12 introns in many of these groups.
U2 and U12 introns were predicted by making use of available EST and genomic sequences. The results show that in species or branches where U12 spliceosomal components are missing, also U12 type of introns are lacking. Examples are the choanoflagellate Monosiga brevicollis, Entamoeba histolytica, green algae, diatoms, and the fungal lineage Basidiomycota. Furthermore, whereas U12 splicing does not occur in Caenorhabditis elegans, U12 introns as well as U12 snRNAs are present in Trichinella spiralis, which is deeply branching in the nematode tree. A comparison of homologous genes in T. spiralis and C. elegans revealed different mechanisms whereby U12 introns were lost.
The phylogenetic distribution of U12 introns and spliceosomal RNAs give further support to an early origin of U12 dependent splicing. In addition, this distribution identifies a large number of instances during eukaryotic evolution where such splicing was lost.
已知有两类内含子，常见的 U2 型和罕见的 U12 型。这两种类型的内含子由不同的剪接体切除。具有 U12 剪接体特征的剪接体 RNA（即 U11、U12、U4atac 和 U6atac RNA）的系统发育分布表明，U12 剪接体在许多系统发育群中丢失。我们现在已经在许多这样的群体中检查了 U2 和 U12 内含子的分布。
利用可用的 EST 和基因组序列预测 U2 和 U12 内含子。结果表明，在 U12 剪接体成分缺失的物种或分支中，也缺乏 U12 型内含子。例如，领鞭毛原生动物 Monosiga brevicollis、溶组织内阿米巴、绿藻、硅藻和真菌 Basidiomycota 谱系。此外，虽然 U12 剪接不在秀丽隐杆线虫中发生，但 U12 内含子和 U12 snRNA 存在于 Trichinella spiralis 中，在线虫树中分支很深。Trichinella spiralis 和 C. elegans 中同源基因的比较揭示了 U12 内含子丢失的不同机制。
U12 内含子和剪接体 RNA 的系统发育分布进一步支持了 U12 依赖性剪接的早期起源。此外，这种分布确定了在真核生物进化过程中，这种剪接丢失的大量实例。
