Sequences homologous to ribosomal insertions occur in the Drosophila genome outside the nucleolus organizer

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Abstract
Many repeating units of D. melanogaster rDNA contain a DNA sequence within the gene for 28S rRNA that does not code for rRNA. This sequence has been called the ribosomal insertion. Members of the same sequence family occur outside the ribosomal locus. Non-rDNA insertion DNA was separated from rDNA by density gradient centrifugation, and sequences homologous to the ribosomal insertion were detected by hybridization with restriction endonuclease fragments derived from a cloned rDNA repeating unit. Pure insertion sequences from cloned rDNA separated from main band DNA and behaved like a component with high G + C [guanine + cytosine] content. Non-rDNA components hybridizing to the insertion also separated from main band DNA but less so than pure insertion sequences, suggesting that non-rDNA insertion sequences are linked to DNA of different nucleotide composition. Restriction endonuclease analysis of non-rDNA insertion DNA showed many fragments of different sizes. The patterns obtained were similar in embryonic, larval, pupal and adult DNA and DNA from cultured cells (Schneider cell line 3). Non-rDNA insertion sequences account for about 0.2% of the genome or about 400 kbases of DNA per haploid complement.