COMPARISON OF 2 ACTIVE HET-A RETROPOSONS OF DROSOPHILA-MELANOGASTER

HeT-A elements are Drosophila melanogaster LINE-like retroposons that transpose to broken chromosome ends by attaching themselves with an ol igo(A) tail. Since this family of elements is believed to be involved in the vital function of telomere elongation in Drosophila, it is impo rtant to understand their transposition mechanism and the molecular as pects of activity. By comparison of several elements we have defined h ere the unit length of HeT-A elements to be approximately 6 kb. Also, we studied an active HeT-A element that had transposed very recently t o the end of a terminally deleted X chromosome. The 12 kb of newly tra nsposed DNA consisted of a tandem array of three different HeT-A eleme nts joined by oligo(A) tails to each other and to the chromosome end b roken in the yellow gene. Such an array may have transposed as a singl e unit or resulted from rapid successive transpositions of individual HeT-A elements. By sequence comparison with another recently transpose d HeT-A element, conserved domains in the single open reading frame (O RF), encoding a gag-like polypeptide, of these elements were defined. We conclude that for transposition an intact ORF is required in cis, w hile the reverse transcriptase is not encoded on the HeT-A element but is provided in tl ans. This would make HeT-A elements dependent on an external reverse transcriptase for transposition and establish contro l of the genome over the activity of HeT-A elements. This distinguishe s the Drosophila HeT-A element, which has been implicated in Drosophil a telomere elongation, from the other, 'selfish' LINE-like elements.