Fig. 9. The major pre-rRNA processing pathway that yields 18 S rRNA in S. cerevisiae.

The 35 S primary transcript contains 18 S, 5.8 S, and 25 S rRNA sequences separated by internal transcribed spacers (ITS1 and ITS2) and flanked by externally transcribed spacers (5′-ETS and 3′-ETS). The processing of the 35 S precursor to mature rRNAs involves endonucleolytic and exonucleolytic steps at the specific sites indicated. Cleavage D occurs in the cytoplasm. The relative positions of oligonucleotides 9–13 are indicated. The pathways leading to 5.8 S and 25 S rRNAs are only briefly summarized here (for a recent review, see Ref. 3).