We conclude this chapter with a brief discussion of transcription initiation by the other two eukaryotic nuclear RNA poly-merases, Pol I and Pol III, and by the distinct polymerases that transcribe mitochondrial and chloroplast DNA. Although these systems, particularly their regulation, are less thoroughly understood than transcription by RNA polymerase II, they are also fundamental to the life of eukaryotic cells.
Transcription Initiation by Pol I and Pol III Is Analogous to That by Pol II
The formation of transcription-initiation complexes involving Pol I and Pol III is similar in some respects to assembly of Pol II initiation complexes (see Figure 11-27). However, each of the three eukaryotic nuclear RNA polymerases requires its own polymerase-specific general transcription factors and recognizes different DNA control elements. Moreover, neither Pol I nor Pol III requires ATP hydrolysis to initiate transcription, whereas Pol II does.
Transcription initiation by Pol I, which synthesizes pre-rRNA, and by Pol III, which synthesizes tRNAs, 5S rRNA, and other short, stable RNAs, has been characterized most extensively in cerevisiae using both biochemical and genetic approaches. It is clear that synthesis of tRNAs and of rRNAs, which are incorporated into ribosomes, is tightly coupled to the rate of cell growth and proliferation. However, much remains to be learned about how transcription initiation by Pol I and Pol III is regulated so that synthesis of pre-rRNA, 5S rRNA, and tRNAs is coordinated with the growth and replication of cells.
Initiation by Pol I The regulatory elements directing Pol I initiation are similarly located relative to the transcription start site in both yeast and mammals. A core element spanning the transcription start site from -40 to +5 is essential for Pol I transcription. An additional upstream element extending from roughly -155 to -60 stimulates in vitro Pol I transcription tenfold.
Pre-rRNA promoter DNA
Upstream activating < factor (UAF)
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