Changes in DNA-dependent RNA polymerase I during ecdysterone induced development of the silkmoth, A. luna



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Texas Tech University


Much of modern biological research focuses on the molecular mechanisms of signal transmission via the membrane or through intracellular compartments which finally influence the pattern and/or intensity of gene expression in the cell nucleus. It is surmised that cis and trans acting factors affect the formation of the preinitiation complexes by the RNA polymerase enzymes. Three distinct and structurally complex classes of RNA polymerases are involved in this process of eukaryotic gene expression called transcription, RNA polymerases I and ill transcribe rRNAs and small RNAs respectively while RNA polymerase II transcribes mRNA.

rRNA synthesis is intimately linked to cell cycle and growth and/or differentiation in response to growth factors and hormones. One example is the tissues of diapausing silkmoths which respond to ecdysterone, the steroid hormone of arthropods, by enhanced RNA synthesis and RNA polymerase activities. The RNA polymerase I activity of the wing epidermis increases nine-fold within 24 hours. The mechanisms for such enhancement, which is seen in almost all instances of stimulated growth, remains unresolved.

An immunological approach to measure the amount of the enzyme based on subunit specific antibodies has been developed. For this purpose, three classes of enzymes were purified from the silkglands. Monoclonal antibodies specific for three subunits of RNA polymerase I (135, 55, and 22 kDa) were produced and utilized to develop a highly reliable, reproducible, and sensitive immunoassay involving antigen capture by immobilized antibodies. Quantitation of the enzyme in ecdysone-stimulated wing epidermal extracts showed increases of only 3-fold in the RNA polymerase I content during the first 8 hours of stimulation and a 4.5-fold increase during the 24-hour time period compared to the nine-fold increase in activity at 24 hours. The covalent modification of RNA polymerase I by phosphorylation as a mechanism to explain the difference has been pursued. Several subunits are phosphorylated at serine and threonine residues which are susceptible to removal by alkaline phosphatase. However, the correlation between increased activity and modification by phosphorylation is equivocal.