Skip to main content
. Author manuscript; available in PMC: 2016 Mar 1.
Published in final edited form as: Annu Rev Biochem. 2015 Feb 20;84:93–129. doi: 10.1146/annurev-biochem-060614-033917

Figure 2.

Figure 2

Cotranscriptional precursor rRNA (pre-rRNA) processing in Saccharomyces cerevisiae. (a) As RNA polymerase I transcribes a ribosomal DNA (rDNA) repeat, nascent pre-rRNAs are cleaved cotranscriptionally at the A2 site, releasing a 43S preribosome containing 20S pre-rRNA. The 66S preribosome containing 27SA2 pre-rRNA is released upon completion of transcription. The pre-rRNA processing sites are indicated along the rDNA gene, and the external and internal transcribed spacer sequences are indicated on the nascent transcript. (b) The pre-rRNAs then undergo a series of exo- and endonucleolytic cleavages to remove the spacer sequences, finally liberating mature 18S, 5.8S, and 25S rRNAs. Not shown is the flanking 5S gene, transcribed in the opposite direction.