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10 RNA Polymerase III Transcription Complexes

E. Peter Geiduschek, George A. Kassavetis


RNA polymerase III (pol III) synthesizes tRNA, 5S ribosomal RNA, and other small RNAs in conjunction with a group of transcription factors (TFIII) that is not yet completely enumerated and characterized. In yeast, TFIIIB serves as the central transcription factor, necessary and sufficient for correctly positioning RNA pol III over the transcriptional start. TFIIIB does not recognize specific DNA sequence, nor is it able to bind to DNA by itself. Instead, it must be directed to its transcription start-proximal upstream DNA-binding site by other TFIIIs acting as assembly factors. The assembly factors of tRNA and 5S RNA genes, TFIIIC and TFIIIA, bind within the transcription unit, with TFIIIA serving as an adapter for TFIIIC in 5S RNA genes. In a U6 snRNA gene, other transcription factors, including the RNA polymerase II transcription factor TFIID, probably assemble TFIIIB from upstream of the transcription start site. It is widely anticipated that these functional principles are generalizable to transcription by RNA pol III in all eukaryotes, but that remains to be proven.

It is 15 years since the publication of the first clear and reliable demonstrations of specifically initiated eukaryotic transcription in vitro by RNA pol III (Parker and Roeder 1977; Wu et al. 1977). The fact that about three-fourths of the chapters of this book deal with eukaryotic transcription reflects (beyond fashion and finance) the enormous development that our understanding of the subject has subsequently undergone. This chapter deals with transcriptional initiation by RNA pol III, which synthesizes small RNAs,...

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