Expression of ribosomal DNA insertions in Drosophila melanogaster

Approximately half of the ribosomal genes on the X chromosome of Drosophila melanogaster are interrupted by an insertion of type 1. Nuclear RNA from D. melanogaster embryos was transferred to DBM paper and hybridized with cloned type 1 insertion sequences. With a DNA fragment derived specifically from large insertions, transcripts were detected between 5 and 10 kb. These insertion transcripts represent less than one RNA molecule per nucleus, which is more than three orders of magnitude below the concentration of nascent rRNA chains, as determined by kinetics of hybridization. With a DNA fragment derived from the right end of large insertions which is also complementary to short insertions, more discrete RNA bands appeared with sizes between 1 and 8.5 kb, representing altogether about 13 RNA molecules per nucleus. Insertion transcripts large enough to be potential precursors to 28S rRNA represent less than one molecule per nucleus. It was shown by sandwich hybridization that at least some of the insertion transcripts are derived from rDNA. No significant difference was found between insertion transcripts in RNA extracted from ovaries, embryos, larvae, pupae or adult flies. Unless a mechanism other than splicing is involved, ribosomal genes with insertions cannot contribute significantly to the synthesis of 28S rRNA. A cytoplasmic RNA approximately 1 kb long, which is complementary to a short insertion and to ribosomal gene sequences flanking both sides of the insertion, was found. The abundance of this short unspliced RNA is about 50 molecules per embryo cell.