Expression of the ribosomal DNA insertions in bobbed mutants of Drosophila melanogaster

We have shown earlier that interrupted rRNA genes in Drosophila melanogaster do not contribute significantly to rRNA production by a splicing mechanism (Long and Dawid 1979). In the work reported here the expression of interrupted rRNA genes was tested in several stocks that carry bobbed mutations, i.e., have partial deletions of their rRNA gene clusters. Transcripts of the major 5 kb type 1 insertion are very rare in bobbed flies as they are in the wild type, occurring at a concentration in embryos of less than one copy per nucleus. Transcripts of short type 1 insertions are more abundant in certain bobbed stocks, especially those carrying the car bb chromosome. However, other severely bobbed flies have no increase in these insertion transcripts over the wild-type levels. Type 2 insertions are transcribed into very rare RNA molecules in the wild type and in the bobbed genotypes that were studied. From these results we conclude that interrupted rRNA genes are not expressed through a splicing mechanism into mature rRNA in mutant or wild-type flies. Since even severely bobbed flies fail to utilize their interrupted rRNA genes, we suggest that these genes cannot be transcribed productively in D. melanogaster.