3RD OPEN READING FRAME OF DROSOPHILA RETROTRANSPOSON MDG4, LIKE RETROVIRAL ENV GENES, IS EXPRESSED THROUGH SPLICING

The presence of a third long open reading frame (ORF3) is a common fea ture of several retrotransposon families, including mdg4 (gypsy). Thus these elements have structural similarity with the integrated forms o f vertebrate retroviruses. To elucidate the mode of ORF3 expression, w e analyzed mdg4 transcription in several Drosophila melanogaster strai ns and in a Schneider 2 cell culture. Subgenomic ORF3 transcripts abou t 2 kb in length were revealed in the fraction of polyadenylated RNAs. PCR amplification of corresponding cDNA molecules showed that these R NAs were generated through splicing of the full-length mdg4 transcript s. In all cases only one DNA fragment was amplified for each suitable pair of primers, indicating that there was no alternative splicing dur ing ORF3 expression. Analysis of the primary structure of spliced RNA showed that in both mdg4 subfamilies (6K and 7K) ORF3 is probably tran slated into functionally active proteins. Regulation at the level of R NA splicing is supposed to be one of important factors affecting trans position frequency of mdg4.