Drosophila melanogaster ribosomal DNA containing type II insertions is variably transcribed in different strains and tissues

Abstract

Unlike the transcripts of the type I ribosomal DNA insertions, transcripts of type II insertions are found only in the nucleus. The most abundant RNA homologous to the type II insertion is 3.4 × 10 3 bases long, corresponding to the length of the most common type II insertion element. There are, in addition, a set of higher molecular weight RNA molecules that consist of insertion transcripts linked mainly to rRNA upstream in the rDNA transcription unit from the insertion. The concentration of the 3.4 × 10 3 base transcript in embryos from our wild-type stock of Drosophila melanogaster (OreR) is about 400-fold lower than the primary transcript of genes without insertions. The concentration of type II transcripts can be up to two orders of magnitude lower in other wild-type strains than in our laboratory stock, or an order of magnitude higher in flies with the y +Y chromosome. The proportion of the higher molecular weight transcripts to the 3.4 × 10 3 base transcripts is much higher in ovarian tissue than in embryos, larvae or pupae.