Institute of Sericulture and Systems Biology, Southwest University, Beibei, Chongqing 400715, China.
J Genet Genomics. 2010 May;37(5):305-14. doi: 10.1016/S1673-8527(09)60048-5.
Eight intact LTR retrotransposons (Nbr1-Nbr8) have been previously characterized from the genome of Nosema bombycis, a eukaryotic parasite with a compact and reduced genome. Here we describe six novel transcribed Nbr elements (Nbr9-Nbr14) identified through either cDNA library or RT-PCR. Like previously determined ones, all of them belong to the Ty3/Gypsy superfamily. Retrotransposon diversity and incomplete domains with insertions (Nbr12), deletions (Nbr11) and in-frame stop codons in coding regions (Nbr9) were detected, suggesting that both defective and loss events of LTR retrotransposon have happened in N. bombycis genome. Analysis of selection showed that strong purifying selection acts on all elements except Nbr11. This implies that selective pressure keeps both these Nbrs and their functions in genome. Interestingly, Nbr11 is under positive selection and some positively selected codons were identified, indicating that new functionality might have evolved in the Nbr11 retrotransposon. Unlike other transposable elements, Nbr11 has integrated into a conserved syntenic block and probably resulted in the inversion of both flanking regions. This demonstrates that transposable element is an important factor for the reshuffling and evolution of their host genomes, and may be maintained under natural selection.
此前已从具有紧凑和简化基因组的真核寄生虫微孢子虫（Nosema bombycis）的基因组中鉴定出 8 个完整的 LTR 反转录转座子（Nbr1-Nbr8）。在此，我们通过 cDNA 文库或 RT-PCR 鉴定了 6 个新的转录 Nbr 元件（Nbr9-Nbr14）。与之前确定的一样，它们均属于 Ty3/Gypsy 超家族。反转录转座子多样性以及插入（Nbr12）、缺失（Nbr11）和编码区中的框内终止密码子（Nbr9）等不完全结构域的存在表明，LTR 反转录转座子的缺失和失活事件均已在 N. bombycis 基因组中发生。选择分析表明，除了 Nbr11 之外，所有元件都受到强烈的纯化选择作用。这意味着选择压力使 Nbrs 及其功能在基因组中得以保留。有趣的是，Nbr11 受到正选择作用，并且鉴定出了一些正选择密码子，表明在 Nbr11 反转录转座子中可能已经进化出了新的功能。与其他转座元件不同，Nbr11 已整合到保守的同线性块中，可能导致侧翼区域的反转。这表明转座元件是其宿主基因组重排和进化的重要因素，并且可能在自然选择下得以维持。
