FIG. 5.

Scheme of pre-rRNA processing in yeast. (A) Structure of the rRNA transcript with the cleavage sites and the location of the hybridization probes. The primary transcript of 35S contains the 5′ external transcribed spacer, followed by the 18S rRNA, the ITS1, the 5.8S rRNA, the ITS2, the 25S rRNA, and the 3′ external transcribed spacer. The mature species of 18S, 5.8S, and 25S are represented by thick lines, and the spacers are represented by thin lines. The location of the three hybridization primers is also shown (Oligo 1, Oligo 2, and Oligo 3). (B) Simplified pre-rRNA processing scheme. The 35S pre-rRNA is cleaved successively at sites A0 and A1 to yield the 33S and 32S pre-rRNA species. Then, cleavage at A2 frees the 20S form that can further be processed into 18S rRNA and the 27SA2 that undergoes a more complex pathway. Cleavage at A3 leads to 27SA3 that can be processed into two different forms. In this simplified scheme, only 27SB is represented that will lead to 7S and 25S after cleavage at C1 and C2. 7S will be further processed into 5.8S.