In vivo transcription of rDNA spacers in Drosophila.

The sequence-nonhomologous rDNA spacers of D. melanogaster and D. virilis exhibit remarkably similar organizations and transcription capabilities. In spacers of both species, tandemly repeated sequences are present upstream of the beginning of the pre-rRNA coding region, and the repeats are capable of transcriptional activity. Short, nuclear transcripts homologous to the spacer repeats have been identified by S1 nuclease protection experiments and by northern blot analyses. Although spacer transcripts are rare in steady state RNA populations, the presence of multiple promoter elements may suggest a regulatory role for rDNA spacers.