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. 2003 Mar;23(5):1798–1807. doi: 10.1128/MCB.23.5.1798-1807.2003

FIG. 1.

FIG. 1.

Pre-rRNA processing pathway. In wild-type cells, the 35S pre-rRNA is cleaved at site A0, producing the 33S pre-rRNA. This molecule is rapidly cleaved at site A1 to produce the 32S pre-rRNA, which is cleaved at site A2 releasing the 20S and 27SA2 pre-rRNAs. The 20S pre-rRNA is exported to the cytoplasm, where it is dimethylated by Dim1p and then cleaved at site D, by an unidentified enzyme, to generate the mature 18S rRNA. 27SA2 is processed via two alternative pathways. It is either cut at site A3 to generate 27SA3, which is then trimmed to site B1S, producing 27SBS. Alternatively, it can be processed to 27SBL by an as yet unknown mechanism. 27SBS and 27SBL are matured to the 5.8S and 25S following identical pathways. Cleavage at site C2 generates the 7S and 26S pre-rRNAs. The 7S pre-rRNA is digested 3′ to 5′ to 6S pre-rRNA and then to the mature 5.8S rRNA. The 26S pre-rRNA is digested 5′ to 3′ to the 25S pre-rRNA and then to the mature 25S rRNA. For a review on pre-rRNA processing and the known processing enzymes, see reference (40).