Burke W D, Eickbush D G, Xiong Y, Jakubczak J, Eickbush T H
Department of Biology, University of Rochester, New York 14627.
Mol Biol Evol. 1993 Jan;10(1):163-85. doi: 10.1093/oxfordjournals.molbev.a039990.
R1 and R2 are retrotransposable elements that integrate at specific sites in the 28S ribosomal RNA (rRNA) genes of Bombyx mori and Drosophila melanogaster. We have previously shown that most insect species contain insertions in their 28S genes at the R1 and/or R2 site. We have sequenced the 3' half of R1 and R2 elements from three additional insect species: the fungus gnat, Sciara coprophila (Diptera); the Japanese beetle, Popillia japonica (Colleoptera); and the parasitic wasp, Nasonia vitripennis (Hymenoptera). The elements were obtained by screening lambda phage genomic clones containing rDNA units and by a polymerase chain reaction approach using degenerate primers to conserved sequences in the reverse-transcriptase domain, in combination with a second primer to the 28S gene 3' of the insertion site. Comparisons of the sequences of R1 and R2 from four insect orders suggest that the organization of their open-reading frames has been conserved and is therefore likely to be similar throughout insects. This sequence analysis also indicates that, except for 5' truncations generated during the retrotransposition process itself, most elements have not accumulated mutations that would make them inactive. Popillia japonica and N. vitripennis differed from previously described species, in that (a) P. japonica contained multiple families of R2 and (b) N. vitripennis contained multiple families of R1. Nucleotide sequence identity between these different families is low. Amino acid sequence identity of their open-reading frames averaged only 41% for the R2 families of P. japonica and 35% for the R1 families of N. vitripennis. The presence of multiple highly divergent families of elements within a species suggests either that each insertion family is able to maintain its copy number without eliminating the other families in its competition for a limited number of 28S genes or that there has been extensive horizontal transfer of R1 and R2 elements between insect species.
R1和R2是逆转座子元件，它们整合在家蚕和黑腹果蝇的28S核糖体RNA（rRNA）基因的特定位点。我们之前已经表明，大多数昆虫物种在其28S基因的R1和/或R2位点存在插入。我们对另外三种昆虫物种的R1和R2元件的3'端进行了测序：粪蝇，嗜粪Sciara（双翅目）；日本甲虫，日本丽金龟（鞘翅目）；以及寄生蜂，丽蝇蛹集金小蜂（膜翅目）。这些元件是通过筛选包含rDNA单元的λ噬菌体基因组克隆，并使用简并引物对逆转录酶结构域中的保守序列进行聚合酶链反应，再结合针对插入位点28S基因3'端的第二种引物而获得的。对来自四个昆虫目的R1和R'序列的比较表明，它们开放阅读框的组织方式是保守的，因此在整个昆虫中可能相似。这种序列分析还表明，除了逆转座过程本身产生的5'端截短外，大多数元件没有积累使其失活的突变。日本丽金龟和丽蝇蛹集金小蜂与先前描述的物种不同，在于：（a）日本丽金龟含有多个R2家族；（b）丽蝇蛹集金小蜂含有多个R1家族。这些不同家族之间的核苷酸序列同一性较低。日本丽金龟R2家族的开放阅读框的氨基酸序列同一性平均仅为41%，丽蝇蛹集金小蜂R1家族的为35%。一个物种内存在多个高度分化的元件家族表明，要么每个插入家族能够在不消除其他家族竞争有限数量28S基因的情况下维持其拷贝数，要么R1和R2元件在昆虫物种之间发生了广泛的水平转移。
