Studies on nucleolar RNA synthesis in Drosophila melanogaster. I. The relationship between number of nucleolar organizers and rate of synthesis.

Nucleolar RNA synthesis in salivary glands from XX, XY, and XO larvae having different numbers of nucleolus organizers was examined autoradiographically following incubation of the tissues in [H]uridine. In addition, the presence or absence of secondary constrictions was monitored in neuroblast preparations using acridine orange staining. It was observed that: (1) The rate of nucleolar [H]uridine incorporation is independent of the number of nucleolus organizers in the cell; (2) nucleolar incorporation in XO tissues is elevated relative to that observed in XX or XY material; and (3) where the number of organizers is in excess of the wild type (2), secondary constrictions form at only 2 of the nucleolus organizer sites. From these and related observations we suggest that there are 2 forms of transcriptional control for nucleolar RNA synthesis. One acts as an on-off control, influencing the formation of secondary constrictions at the nucleolus organizers. The other form modulates rates of transcription at organizers where constrictions have been formed.