Transcriptional organization of the Drosophila melanogaster ribosomal RNA genes

Nuclear RNA transcripts from Drosophila melanogaster ribosomal RNA genes containing introns

The transcription of ribosomal genes in a cell line (Kc) derived from female D.melanogaster, has been investigated using hybridization probes prepared from restriction fragments of a cloned rDNA repeat with a 5 kb type I [Wellauer et al. (1978) Cell 14, 269-278] intron. Gels, of nuclear RNA that have been transferred to diazotized paper and hybridized with labelled intron sequences, reveal both large (1-10 kb) transcripts and a discrete 325 base species. Berk-Sharp experiments [(1977) Cell 12, 721-732] reveal large transcripts that are homologous to intron sequences and extend into 28S sequences as well. However, while the abundance of 28S transcripts is about 50,000 copies per nucleus [Clark et al. (1977) Genetics 86, 789-800], these long transcripts are only present at 1-2 copies per nucleus and the 325 base species is only 10 times more abundant. In view of the fact that female cells have about 400 rDNA genes, 49% of which have type I introns, one must conclude either that transcription rarely occurs from the genes containing introns (the majority) or these transcripts are processed unusually rapidly. Transcripts homologous to the "non-transcribed spacer" region have been found, but their abundance is no higher.