Two variants of the Drosophila melanogaster retrotransposon gypsy (mdg4): structural and functional differences, and distribution in fly stocks

Two variants of the Drosophila melanogaster retrotransposon gypsy were subj ected to detailed structural and functional analysis. A series of hybrid co nstructs containing various combinations of "active" and "inactive" gypsy c opies were tested for their ability to produce new DNA copies in cultured c ells by means of reverse transcription. It was shown that the previously de monstrated variations in retrotranspositional activity are associated with either one or both of two amino acid substitutions at the beginning of ORF2 . The first substitution is located at the boundary between the putative pr otease and reverse transcriptase domains and, hence, may influence the proc essing of the polyprotein. The other substitution may alter reverse transcr iptase activity since it is located in the second of the seven conserved do mains of the RT gene. To address the question of the evolutionary relations hip between the two gypsy variants, their distribution was analyzed in amon g various fly stocks. Southern analysis revealed that all D. melanogaster s trains studied so far contain the "inactive" gypsy variant, while the ''act ive" copies are present only in some strains; most of the latter were estab lished from flies recently isolated from natural populations. Finally, in s tocks carrying the flamenco mutation the "active" gypsy variant is much mor e abundant than the "inactive" form. Possible scenarios for the origin of t he "active" form of gypsy are discussed.