RNA polymerase III transcription machinery: Structure and transcription regulation

RNA polymerase (Pol) III is a complex multisubunit enzyme that transcribes genes for small stable untranslated RNAs (tRNAs, 5S rRNA, Alu RNA, U6 snRNA) and some other genes (class III genes). Of all eukaryotic Pols, Pol III is the most poorly studied. Recent attention to Pol III is explained by new data on the important role of its RNA products in cell growth, proliferation, and differentiation. The contents of these RNA products depend on the growth rate, the cell-cycle phase, and the physiological state of the cell (slow or active proliferation or apoptosis) and change upon malignant transformation, viral infection, and heat shock. This review considers the structures and functions of Pol III, its basal transcription factors, and the holoenzyme; assembly of the preinitiation complex; and the transcription cycle. In addition, transcriptional regulation of class III genes is described as dependent on the cell cycle phase, growth factors, cell growth rate, transformation, and apoptosis.