Transient expression of Drosophila melanogaster rDNA promoter into cultured Drosophila cells.

Recombinant plasmids that carry the bacterial CAT gene under the transcriptional control of the D. melanogaster rDNA promoter have been introduced by transfection into cultured Schneider II Drosophila cells and their template activity followed at the RNA and protein level. While no CAT enzyme activity is measurable 48 hrs after transfection, high levels of hybrid rRNA-CAT transcripts that originate at the authentic rRNA start site are detected by S1 mapping analysis. The interval -180/+34 of a rDNA transcriptional unit is sufficient to ensure faithful polymerase I transcription. However, the presence of a complete NTS (non transcribed spacer) region greatly enhances the transcriptional activity of exogenously added rDNA templates. Competition experiments between constructs carrying different amounts of NTS sequences indicate that spacer segments confer a transcriptional advantage efficiently attracting necessary transcription factors and/or polymerase I molecules.