[Expression of the third open reading frame of the drosophila MDG4 retrotransposon similar to the retroviral env-genes, occurring through splicing].

The presence of a third long open reading frame (ORF3) is the common feature of a number of Drosophila retrotransposons, including MDG4 (gypsy). Thus, these elements have a strong structural resemblance to the integrated forms of vertebrate retroviruses. To elucidate the mode of expression of ORF3, transcription analysis of MDG4 for several D. melanogaster strains and Schneider 2 cultured cells was carried out. In all cases the analysis revealed the presence of 2-kb subgenomic ORF3 transcripts in the polyadenylated RNA fractions. It was shown by using the cDNA-PCR technique with different sets of primers that these subgenomic 2-kb RNAs are generated through splicing of full-length MDG4 transcripts. In all cases, only one DNA fragment is amplified for each suitable pair of primers, indicating that there is probable no alternative splicing during the ORF3 expression. The primary structure of spliced RNA was determined, and its analysis shows that both MDG4 subfamilies (6K and 7K) are apparently able to encode functionally active translation products of ORF3. The regulation at the level of splicing is supposed to be one of the most important factors controlling the transposition frequency of MDG4.