Volff J N, Hornung U, Schartl M
Physiologische Chemie I, Biozentrum, University of Würzburg, Germany.
Mol Genet Genomics. 2001 Jun;265(4):711-20. doi: 10.1007/s004380100468.
Poseidon and Neptune are two ancient lineages of retroposons related to the Penelope element from Drosophila virilis. They have been identified in various teleost fish species, including the medakafish (Oryzias latipes), and the pufferfishes Fugu rubripes and Tetraodon nigroviridis, whose genomes are currently being sequenced. Some of these elements are highly reiterated in fish genomes. Penelope-related elements were also identified in blood fluke, shrimp, sea urchin, cichlid fish and frog, showing that they are widespread in animals. Penelope-related retroposons were not detected among sequences from the Drosophila melanogaster and human genome projects, suggesting that they have been lost from certain animal lineages. A sequence encoding a putative Uri (also called GIY-YIG) endonuclease domain was detected downstream from the gene for reverse transcriptase. To the best of our knowledge, this type of endonuclease sequence has previously been identified in group I introns and in genes for prokaryotic excinucleases but not in retrotransposable elements. Penelope-related elements are frequently truncated at their 5' ends and can also be flanked by long terminal repeat-like structures. Phylogenetic analysis of the reverse transcriptase domain failed to assign Penelope-related retroposons to one of the major groups of retroelements. Overall, therefore, the evidence strongly suggests that these sequences represent a new group of retrotransposable elements.
波塞冬（Poseidon）和海王星（Neptune）是与果蝇（Drosophila virilis）的佩内洛普元件（Penelope element）相关的两个古老反转录转座子谱系。它们已在多种硬骨鱼物种中被鉴定出来，包括青鳉（Oryzias latipes）、河豚（Fugu rubripes）和暗绿鲀（Tetraodon nigroviridis），这些物种的基因组目前正在进行测序。其中一些元件在鱼类基因组中高度重复。在血吸虫、虾、海胆、丽鱼科鱼类和青蛙中也鉴定出了与佩内洛普相关的元件，这表明它们在动物中广泛存在。在黑腹果蝇（Drosophila melanogaster）和人类基因组计划的序列中未检测到与佩内洛普相关的反转录转座子，这表明它们已在某些动物谱系中丢失。在逆转录酶基因下游检测到一个编码假定的Uri（也称为GIY - YIG）内切核酸酶结构域的序列。据我们所知，这种类型的内切核酸酶序列以前在I组内含子和原核生物切除核酸酶的基因中被鉴定出来，但在反转录转座元件中未被发现。与佩内洛普相关的元件在其5'端经常被截断，并且也可能侧翼有类似长末端重复的结构。对逆转录酶结构域的系统发育分析未能将与佩内洛普相关的反转录转座子归入反转元件的主要类别之一。因此，总体而言，有力的证据表明这些序列代表了一类新的反转录转座元件。
