Close relationship between the long terminal repeats of avian leukosis-sarcoma virus and copia-like movable genetic elements of Drosophila.

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Abstract
A new species of copia-like movable genetic element termed 17.6 was identified in Drosophila melanogaster, and the nucleotide sequences of its long terminal repeats (LTRs) were determined. The LTRs of 17.6 were not only homologous to those of 297, a sibling movable genetic element of 17.6, but also closely matched those of avian leukosis-sarcoma virus. This made it possible (i) to identify the nucleotide sequences in 17.6 and 297 that correspond to the crucial regulatory sequences for both transcription and reverse transcription in avian leukosis-sarcoma virus and (ii) to divide the LTRs of these two elements into three regions, U3, R, and U5, like those of retrovirus proviruses. Similarity in sequence was also found to a certain extent in other copia-like elements. From these results, we postulate that copia-like movable genetic elements in Drosophila originated from infection of a progenitor Drosophila with a retrovirus from which the present-day avian leukosis-sarcoma virus was derived.