Malik H S, Henikoff S, Eickbush T H
Fred Hutchinson Cancer Research Center, Seattle, Washington 98109 USA.
Genome Res. 2000 Sep;10(9):1307-18. doi: 10.1101/gr.145000.
Phylogenetic analyses suggest that long-terminal repeat (LTR) bearing retrotransposable elements can acquire additional open-reading frames that can enable them to mediate infection. Whereas this process is best documented in the origin of the vertebrate retroviruses and their acquisition of an envelope (env) gene, similar independent events may have occurred in insects, nematodes, and plants. The origins of env-like genes are unclear, and are often masked by the antiquity of the original acquisitions and by their rapid rate of evolution. In this report, we present evidence that in three other possible transitions of LTR retrotransposons to retroviruses, an envelope-like gene was acquired from a viral source. First, the gypsy and related LTR retrotransposable elements (the insect errantiviruses) have acquired their envelope-like gene from a class of insect baculoviruses (double-stranded DNA viruses with no RNA stage). Second, the Cer retroviruses in the Caenorhabditis elegans genome acquired their envelope gene from a Phleboviral (single ambisense-stranded RNA viruses) source. Third, the Tas retroviral envelope (Ascaris lumricoides) may have been obtained from Herpesviridae (double-stranded DNA viruses, no RNA stage). These represent the only cases in which the env gene of a retrovirus has been traced back to its original source. This has implications for the evolutionary history of retroviruses as well as for the potential ability of all LTR-retrotransposable elements to become infectious agents.
系统发育分析表明，携带长末端重复序列（LTR）的逆转座元件可获得额外的开放阅读框，从而使其能够介导感染。虽然这一过程在脊椎动物逆转录病毒的起源及其包膜（env）基因的获得方面有最充分的记录，但类似的独立事件可能也发生在昆虫、线虫和植物中。env样基因的起源尚不清楚，而且常常被最初获得这些基因的古老性及其快速的进化速度所掩盖。在本报告中，我们提供证据表明，在LTR逆转座子向逆转录病毒的另外三种可能的转变中，一个包膜样基因是从病毒来源获得的。首先，gypsy及相关的LTR逆转座元件（昆虫errantiviruses）从一类昆虫杆状病毒（无RNA阶段的双链DNA病毒）获得其包膜样基因。其次，秀丽隐杆线虫基因组中的Cer逆转录病毒从静脉病毒（单链双义RNA病毒）来源获得其包膜基因。第三，Tas逆转录病毒包膜（蛔虫）可能是从疱疹病毒科（无RNA阶段的双链DNA病毒）获得的。这些是逆转录病毒的env基因被追溯到其原始来源的仅有的例子。这对逆转录病毒的进化史以及所有LTR逆转座元件成为感染因子的潜在能力都有影响。
