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. 1997 Dec 9;94(25):13536–13541. doi: 10.1073/pnas.94.25.13536

Figure 1.

Figure 1

Pre-rRNA processing in S. cerevisiae. (A) The nascent 35S rRNA transcript undergoes successive cleavages at the A0, A1, and A2 sites, dividing it into precursors to the small and large subunit rRNAs. The 20S is processed into the small subunit 18S rRNA. The 27SA2 precursor gives rise to the large subunit rRNAs, 5.8S and 25S. The large subunit RNAs can also be processed from the 27SB precursor, produced by either exonucleolytic degradation to B1 or by direct cleavage at B1. The 27SB precursor undergoes further processing steps to form the mature large subunit rRNAs. (B) Pre-rRNA processing in S. cerevisiae expressing truncated Mpp10 proteins. The levels of rRNA precursors that are generated by cleaving at A1 and A2, the 32S, 20S, and 27SA2 pre-rRNAs, are reduced, whereas the levels of 33S and 23S precursors are similar. A new 22S precursor is visible, resulting from competent cleavage at A0 and deficient cleavage at A1 and A2. It is not known if the 22S precursor is degraded or is slowly processed to form the 18S rRNA.