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. 2000 Oct;20(19):7238–7246. doi: 10.1128/mcb.20.19.7238-7246.2000

FIG. 1.

FIG. 1

Pre-rRNA processing pathway in yeast. (A) Structure of the rRNA gene operon and location of the oligonucleotides used in this work. The mature 18S, 5.8S, and 25S rRNAs (bold lines) are released from the 35S primary transcript following cleavages in the 5′-ETS and 3′-ETS and ITS1 and ITS2. Cleavage sites are indicated by uppercase letters (A0 to D). Oligonucleotides used for the Northern blot hybridization and primer extension experiments are indicated by lowercase letters (a to h). (B) Pre-rRNA processing pathway. The 35S pre-rRNA is successively cleaved in the 5′-ETS at site A0 (generating the 33S RNA), at site A1, the 5′ end of the mature 18S rRNA (generating the 32S RNA), and at site A2 in ITS1 (generating the 20S and 27SA2 pre-rRNAs). The 20S is dimethylated by Dim1p and cleaved at site D to generate the mature 18S rRNA. The 27SA2 is matured to the 5.8S and 25S rRNAs following two alternative pathways. Eighty-five percent of the 27SA2 population is cleaved at site A3 in ITS1 by RNase MRP; this is rapidly followed by 5′-to-3′ trimming to site B1S by Xrn1p and Rat1p. Fifteen percent of 27SA2 is cleaved at site B1L. Cleavage at site B2 occurs concomitantly with cleavage at site B1. The two forms of 27SB (27SBS and 27SBL) are matured following identical pathways involving cleavage at sites C1 and C2 and 3′-to-5′ exonucleolytic digestion to site E by the exosome. Both Dhr1p and Dhr2p are required for cleavages at sites A1 and A2; Dhr2p is also required for cleavage at site A0. Base pairing between U3 and the pre-rRNA in the 5′-ETS and 18S regions is required for cleavage at sites A1 and A2; the U3–5′-ETS interaction is also required for cleavage at site A0. (C) Structure of aberrant pre-rRNA processing intermediates detected on depletion of Dhr1p and Dhr2p. Premature cleavage of the 35S pre-rRNA at site A3 in ITS1 generates the 23S RNA. The 22.5S RNA results from cleavage of pre-rRNA molecules at site D in the absence of cleavage at sites A0 and A1. The 22S RNA is accumulated when cleavages occur at sites A0 and A3 in the absence of cleavage at sites A1 and A2.