Ribosomal insertion-like elements in Drosophila melanogaster are interspersed with mobile sequences

Sequences homologous to the type 1 ribosomal DNA insertion occur in the chromocenter and in one location on the fourth chromosome of Drosophila melanogaster. Ribosomal insertion-like elements frequently occur in the form of tandem repeats and are often interspersed with nonhomologous DNA. Sequences that interrupt of flank insertion-like elements are members of diverged repeated DNA families. Members of five of these interspersed sequence families are located at multiple euchromatic sites as well as in the chromocenter. The euchromatic sites differ greatly in the gt-1 and gt-X11 stocks of D. melanogaster, suggesting that these sequence families are mobile in the genome. No long inverted repeats were detected in any of these interspersed sequences. One sequence, called 101F, interrupts a ribosomal insertion-like element; the nucleotide sequences across the boundaries between 101F and ribosomal insertion have been determined. A stretch of 13 base pairs that is present once in the ribosomal insertion is repeated at the ends of the 101F sequence, suggesting a target site duplication. Within the 101F element, no extended direct or inverted repeat sequence exists. The interspersed repeated sequences studied are transcribed into rare, heterogeneous, poly(A)-lacking nuclear RNA molecules. In one case, we showed that both strands of a flanking sequence are transcribed, but to a different extent. With respect to structural features and the nature of their transcripts, the sequences described here are distinct from other known transposable elements in Drosophila.