The largest group of transposable elements in Drosophila melanogaster, copia-like elements, share some important structural features with and are intimately related in evolution to vertebrate retroviruses. To further clarify the relationship between retroviruses and copia-like transposable elements, we set out to determine the complete nucleotide sequence of the genome of 17.6, which has long terminal repeats homologous in nucleotide sequence to those of avian leukaemia-sarcoma virus. We report here that 17.6 contains three long open reading frames comparable with gag, pol and env genes in retrovirus. At the level of amino acid sequence, the longest open reading frame of 17.6 includes a coding sequence similar to that for reverse transcriptase, suggesting a role for this enzyme in the life cycle of some Drosophila copia-like elements, analogous to the situation in retrovirus.

We have made a computer-assisted search for homology among polymerases or putative polymerases of various viruses and a transposable element, the Drosophila copia-like element 17.6. The search revealed that the putative polymerase (second open reading frame) of the copia-like element 17.6 bears close resemblance in overall structural organization to the pol gene product of Moloney murine leukaemia virus (M-MuLV): they show significant homology to each other at both the Nand C-terminal portions, suggesting that the 17.6 putative polymerase carries two enzymatic activities, related to reverse transcriptase and DNA endonuclease. The putative polymerase of cauliflower mosaic virus (CaMV) shows striking homology with the putative polymerase of 17.6 over almost its entire length, but it lacks the DNA endonuclease-related sequence. Furthermore, it was shown that the N-terminal ends of the M-MuLV pol product and the CaMV and 17.6 putative polymerases exhibit strong sequence homology with the gag-specific protease (p15) of Rous sarcoma virus (RSV) as well as the amino acid sequence predicted from the gag/pol spacer sequence of human adult T-celi leukaemia virus (HTLV). These p15-related sequences contain a highly conserved stretch of amino acids which show a close similarity with sequences around the active site amino acids Asp-ThrGly of the acid protease family, suggesting that they have an activity similar to acid protease. On the basis of the alignment of reverse transcriptase-related sequences, a dendrogram representing phylogenetic relationships among all the viruses compared together with 17.6 was constructed and its evolutionary implication is discussed.

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.In Drosophila melanogaster, about 5% of the genome DNA is formed of copia-like movable genetic elements scattered at numerous sites along the chromosome (1). Recent analyses have revealed that, like retrovirus proviruses in vertebrates, the copia-like elements not only are bounded by long terminal direct repeats (LTRs) but also are flanked by short direct repeats of host DNA (1-11). In addition, nucleotide sequences seemingly corresponding to a primer binding site or a purine-rich sequence (or both), both of which appear to be mandatory for the initiation of DNA synthesis in a retrovirus system (4, 12, 13),

The genome of D. melanogaster has been shown to conatin a large group of retrotransposons, which are related in evolution to retroviruses and suggested to be reverse transcribed on translocation(l). The evolutionary rate of any RNA genomes including retrovirus is suggested to be more than a million times greater than that of DNA genomes(2). Thus,if retrotransposons frequently move on the chromosome, one may expect the evolutionary rate of retrotransposons to be much higher than that of normal genes. To * *