Lerat E, Capy P
Laboratoire Populations, Génétique et Evolution, Centre National de la Recherche Scientifique, Gif-sur-Yvette, France.
Mol Biol Evol. 1999 Sep;16(9):1198-207. doi: 10.1093/oxfordjournals.molbev.a026210.
Retroviruses and long terminal repeat (LTR) retrotransposons share a common structural organization. The main difference between these retroelements is the presence of a functional envelope (env) gene in retroviruses, which is absent or nonfunctional in LTR retrotransposons. Several similarities between these two groups of retroelements have been detected for the reverse transcriptase, gag, and integrase domains. Assuming that each of these domains shares a common ancestral sequence, several hypotheses could account for the emergence of retroviruses from LTR retrotransposons. In this context, the positions of elements such as gypsy and the members of the Ty3 subfamily are not clear, since they are classified as retroviruses but phylogenetically they are assigned to the LTR retrotransposon group. We compared the env gene products of these retroelements and identified two similar motifs in retroviruses and LTR retrotransposons. These two regions do not occur in the same order. If we assume that they are derived from the same ancestral sequence, this could result from independent acquisition of the various domains rather than the single acquisition of the whole env gene. However, we cannot exclude the possibility that the env gene was reorganized after being acquired. Trees based on these regions show that these two groups of elements are clearly distinguished. These trees are similar to those obtained from reverse transcriptase or integrase. In trees based on reverse transcriptase, the retroviruses with complete or partial env genes can be distinguished from the other LTR retrotransposons.
逆转录病毒和长末端重复序列（LTR）逆转座子具有共同的结构组织。这些逆转元件之间的主要区别在于逆转录病毒中存在功能性包膜（env）基因，而在LTR逆转座子中该基因不存在或无功能。在逆转录酶、gag和整合酶结构域方面，已检测到这两组逆转元件之间存在若干相似之处。假设这些结构域中的每一个都共享一个共同的祖先序列，那么有几种假说可以解释LTR逆转座子如何产生逆转录病毒。在这种情况下，诸如gypsy元件和Ty3亚家族成员等元件的位置并不明确，因为它们被归类为逆转录病毒，但在系统发育上它们被归为LTR逆转座子组。我们比较了这些逆转元件的env基因产物，并在逆转录病毒和LTR逆转座子中鉴定出两个相似的基序。这两个区域的排列顺序不同。如果我们假设它们源自相同的祖先序列，这可能是由于各个结构域的独立获得，而不是整个env基因的单次获得。然而，我们不能排除env基因在获得后发生重组的可能性。基于这些区域构建的系统发育树表明，这两组元件明显不同。这些树与从逆转录酶或整合酶获得的树相似。在基于逆转录酶构建的树中，具有完整或部分env基因的逆转录病毒可以与其他LTR逆转座子区分开来。
