A family of oligo-adenylate-terminated transposable sequences in Drosophila melanogaster.

Five members of the F family of transposable elements in Drosophila melanogaster have been characterized. Together with earlier work, these experiments lead to the following conclusions. There are about 50 copies of F elements in the genome located at about 25 euchromatic sites and in the chromocenter. Three of the cloned F elements have a length of 4.7 X 10(3) bases, the other two are truncated at their left ends by 0.4 X 10(3) and 1.3 X 10(3) bases, the other two are truncated at their left ends by 0.4 X 10(3) and 1.3 X 10(3) bases. The shortest F element has been found in a recently arisen revertant of the white ivory mutation. All F elements terminate at their right ends in a stretch of 12 to 30 A residues. A polyadenylation signal (A-A-T-A-A-A) precedes this terminus. Analysis of homologous DNA regions derived from two different strains of D. melanogaster containing F elements (filled sites) or lacking F elements (empty sites) showed that F insertions lead to the generation of target site duplications of 8 to 13 base-pairs. The five target sites analyzed differ in sequence but show a low level of homology. F elements do not contain internal repeats and, in particular, they lack the terminal redundancy characteristic of many other transposable elements. We compare F elements with other oligo(A)-terminated sequences, specifically the Alu sequences and processed pseudogenes of mammals. The presence of an oligo(A) stretch and of a polyadenylation signal raises the possibility that F elements transpose by a mechanism that involves transcription and reverse transcription followed by reinsertion into the genome.