The Yeast RNA Polymerase III Transcription Machinery: A Paradigm for Eukaryotic Gene Activation

Since the discovery of an RNA polymerase activity inrat liver nuclei four decades ago, each successive phase inthe development of this field has revealed additional subtlety and complexity of the transcription apparatus. Thethree forms of nuclear RNA polymerase have been purified from a variety of eukaryotic organisms, and in vitrotranscription studies have conclusively established theirgene specificity: RNA polymerase I (pol I) synthesizes ribosomal RNA precursors, whereas RNA polymerase II(pol II) transcribes protein-encoding genes and RNApolymerase III (pol III) synthesizes small RNAs (mostly5S RNA and tRNA) (Sentenac 1985). The availability ofcell-free systems capable of accurately transcribing exogenous DNA opened the way to the identification of theDNA elements, promoters, enhancers, and other regulatory signals that direct and regulate accurate initiation oftranscription, and this, in turn, encouraged a generalsearch for specific DNA-binding proteins. The harvestwas plentiful. In yeast, genetic and biochemical approaches have already identified 80 dedicated transcription factors (Svetlov and Cooper 1995), but analysis ofthe complete genome sequence suggests that even asmany as 6% of all yeast genes may be involved in transcription regulation. The identification of the generaltranscription factors (GTFs) for all three reconstitutedtranscription systems was less straightforward and constitutes in itself a remarkable achievement. The entire setof GTFs comprises approximately 50 polypeptides, twothirds of which are devoted to pol II (Orphanides et al.1996). The in vitro sequential assembly of the GTFs intopreinitiation complexes gave insights into the functionand interaction properties of the different components.However, this stepwise assembly model was challengedby the discovery of preassembled pol II holoenzymescontaining both several GTFs and other novel factors(Koleske and Young 1995). The primary step of gene activation could be considered as the recruitment of aholoenzyme at the promoter under the combinatorial control of gene-specific transcription factors...