Ecdysone Regulated RNA synthesis in Drosophila larval salivary glands

The changes in total and ribosomal RNA (rRNA) synthesis in cultured larval salivary glands of Drosophila in response to steroid hormone ecdysone have been studied. By using the incorporation of [3H]-uridine or [32P]-UTP into salivary gland cells, followed by RNA extraction and electrophoretic or sucrose gradient fractionation it was found that physiological concentrations of ecdysone stimulate the synthesis of rRNA. Administration of selective transcription inhibitors (actinomycin D, alpha-amanitin, cordycepin, DRB, and 5- fluorouridine) as well as protein synthesis inhibitors (cycloheximide, puromycin) showed that early response to ecdysone involved a direct stimulation of rDNA transcription, probably by ecdysone-ecdysone receptor complex, and is independent of protein synthesis. In later stages of the response to ecdysone, as revealed again using transcription and protein synthesis inhibitors, the synthesis of rRNA is dependent on the factor(s) produced during early stages of the response, in the manner which is very similar to sequential gene activation in chromosomal puffs. Experiments with the withdrawal of ecdysone from culture medium in early stages of its action show similar negative effects on rRNA synthesis as inhibitors of RNA polymerase II, thus proving the existence of parallel mechanism controlling induction of specific chromosomal puffs and transcription of rDNA. The data presented suggest that, in addition to the expression of tissue and developmentally-specific genes, the regulation of rDNA transcription, along with the production of ribosomes, might be required for Drosophila metamorphosis. The results are discussed in the light of vertebrate's steroid hormone action on ribosomal genes.