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The synthesis of ribosomes is an essential cellular process which requires the transcription of the rRNA genes by RNA polymerase I (Pol I). The regulation of rRNA synthesis is known to be coupled to growth regulation.In nongrowing, slowly growing, and rapidly growing Drosophila cells, exposure to the tumor-promoting phorbol ester 12-0-tetradecanoylphorbol-13-acetate (TPA) increases the synthesis of precursor and mature rRNAs. Using nuclear run-on assays, we show that TPA enhances transcription of the rRNA genes. These results suggest that TPA regulates expression of rRNA genes transcribed by Pol I, irrespective of the growth state of the cells. In slowly dividing Drosophila cells, increasing the serum concentration rapidly alters the accumulation of rRNA by enhancing rDNA transcription within 1 h. Thus, TPA and serum are each able to rapidly regulate rRNA gene expression in Drosophila cells. These results indicate that the RNA Pol I transcription system can be regulated by agents which have previously been shown to effect specific genes transcribed by the RNA Pol II system.It is now widely accepted that tumor-promoting phorbol esters, which mimic the action of diacylglycerol, can activate a calcium-and phospholipid-dependent protein kinase, protein kinase C (reviewed in references 4, 17, and 20). We have previously observed that protein synthesis can be stimulated within minutes in nondividing Drosophila cells by reagents that act through a pathway involving calcium and the activation of protein kinase C (32). We found that phorbol esters stimulated protein synthesis and that this stimulation was dependent on the presence of calcium, suggesting a common signaling pathway. Since in most systems, the cellular processes of protein and rRNA synthesis are closely coupled (reviewed in reference 21), we have investigated whether in nondividing cells, activation of protein kinase C by 12-O-tetradecanoylphorbol-13-acetate (TPA) can bring about changes in rRNA synthesis parallel to its affects on protein synthesis.Since TPA is known to affect growth-related events in dividing cells, we also chose to investigate the effects of this reagent on rRNA synthesis in dividing Drosophila cultured cells which were either growing slowly, i.e., under serumreduced conditions, or growing rapidly, in the presence of a high concentration of serum.There are several levels at which stimulation or inhibition of RNA synthesis could potentially be controlled. We have investigated the changes in rDNA transcription which take place in Drosophila cells treated with TPA and serum by using nuclear run-on assays to examine the number of active polymerase I (Pol I) complexes present on the 18S and 28S rRNA genes. MATERIALS AND METHODS Drosophila culture and male accessory gland isolation.Wild-type Drosophila melanogaster (Oregon-R) was grown at 25°C on a standard cornmeal-Karo-yeast agar medium supplemented with live yeast cells. After eclosion, the flies were separated according to sex. Accessory glands from 10 virgin males (7 to 10 days old) were ...

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