FIG. 1.
Structure and relevant processing steps of the 35S pre-rRNA in S. cerevisiae. In the 35S pre-rRNA, sequences of the mature 18S, 5.8S, and 25S rRNAs are separated by two internal transcribed spacers (ITS1 and ITS2) and flanked by 5′ and 3′ external transcribed spacers. The 5′ ETS is removed by sequential cleavages at A0 and A1 to generate 32S pre-rRNA. Processing of ITS1 and ITS2 is more complex. Cleavage of 32S pre-rRNA at A2 separates 20S pre-rRNA from 27SA2 pre-rRNA. Cytoplasmic processing of 20S pre-rRNA to mature 18S rRNA occurs by cleavage at D. 5′ processing of 27SA2 pre-rRNA follows alternative pathways. Approximately 90% of 27SA2 pre-rRNA undergoes cleavage at A3, producing 27SA3, which is subsequently processed at B1S to produce 27SBS. The remaining 10% is processed at B1L, which produces 27SBL pre-rRNA. At the stage of 27S pre-rRNA, the mature 3′ end of 25S rRNA is formed by cleaving 27S pre-rRNAs at B2. Subsequent processing of 27SBS and 27SBL precursors is identical. Cleavage of these precursors at C2 and C1 releases 25S rRNA and two forms of 7S pre-rRNA, 7SS and 7SL, respectively. The mature end of 5.8SS and 5.8SL rRNAs are produced by 3′→5′ digestion to site E of 7S pre-rRNAs. The longest form, 5.8SL rRNA, contains a 5′ 6- to 8-nucleotide extension. Letters below the 35S pre-rRNA diagram (probes A to F) indicate the positions of the oligonucleotide probes used in Northern blot analysis.