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12 Regulation of Transcription by RNA Polymerase I

Ronald H. Reeder


RNA polymerase I (pol I) transcribes ribosomal genes, which often are tandemly repeated with two transcription units for pol I per repeating unit of ribosomal DNA (rDNA). One transcription unit produces ribosomal precursor RNA, whereas the other produces unstable transcripts across the pol I enhancers in the intergenic spacer. The pol I enhancers share several fundamental properties with pol II enhancers.

In the vertebrates, two accessory factors, UBF and SL1, cooperate to form a stable promoter initiation complex that is recognized by pol I. UBF has been cloned, revealing that it contains DNA-binding domains related to a domain present in the chromosomal proteins, HMG1 and HMG2. Short-term regulation of rRNA transcription occurs by altering the ability of the polymerase to initiate at the stable promoter complex.

In this paper, I review the cis- and trans-acting elements that regulate the initiation of transcription by RNA pol I and the mechanisms by which they operate. Pol I is only known to produce two transcripts in the eukaryotic cell: One transcript is the precursor to the stable 18S, 5.8S, and 28S rRNAs of cytoplasmic ribosomes and accounts for one-third to one-half of nuclear RNA synthesis. Regulating the synthesis of this transcript is an important mechanism in controlling the overall abundance of ribosomes, and it has long been recognized that the cytoplasmic content of ribosomes correlates with the growth rate of both prokaryotes (Maaloe and Kjeldgaard 1966) and eukaryotes (Waldron and Lacroute 1975; Waldron 1977). In many cells, pol I also produces...

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