Transcription of the tandem array of ribosomal DNA in Drosophila melanogaster does not terminate at any fixed point.

Transcription termination of the polymerase I transcribed rRNA genes is thought to occur at or closely behind the end of the 28S gene coding region. We show here that this is not the case for Drosophila melanogaster. Nuclease S1 mapping of total RNA and a nuclear ;run-on' assay reveal that there is no defined point of termination throughout the whole spacer separating the two genes. Since the rRNA genes in D. melanogaster are organized in tandem arrays, this implies that readthrough transcription of one unit can occur up to, and possibly through, the promoter of the next unit. Readthrough transcription might be a component of transcription enhancement in a tandem array of genes.