Hormonal regulation of transcription of rDNA. Purification and characterization of the hormone-regulated transcription factor IC

Glucocorticoids reversibly inhibit transcription of ribosomal RNA genes in murine lymphosarcoma P1798 cells in culture. Inhibition of rDNA transcription is due to reduction in the amount or activity of an RNA polymerase I transcription factor called transcription factor IC (TFIC). TFIC has been purified over 100,000-fold. The highly purified preparation contains neither RNA polymerase I activity nor any of the conventional RNA polymerase I subunits. TFIC activity co-purifies with three polypeptides of approximately 55, 50, and 42 kDa molecular mass. These polypeptides are present in a stoichiometric ratio of 1:1:1.