FIGURE 9-51 Transcription of the rRNA precursor RNA by RNA polymerase I. (Top) Electron micrograph of RNA-protein complexes transcribed from one copy of the repeated rRNA genes. (Middle) A single Pol I transcription unit. Enhancers that stimulate Pol I transcription from a single transcription start site are represented by blue boxes. Pol I transcription termination sites (T₀, T₁–T₁₀) bound by the Pol I–specific termination factor TTF-1 are shown as red rectangles. pRNA indicates transcription of the noncoding pRNA required for transcriptional silencing. The sequences of regions of DNA shown as yellow rectangles are retained during processing of 18S, 5.8S and 28S rRNAs. The other regions transcribed from the black arrow to the red termination sites are removed and degraded. (Bottom) The core promoter element and upstream control element are shown with the location of Pol I and its general transcription factors UBF, SL1, and TIF-1A represented, as well as other proteins required for Pol I elongation and control. See I. Grummt, 2010, FEBS J. 277:4626.

[Electron micrograph courtesy Ann L. Beyer.]