Figure 3.

35S Pre-rRNA Processing Pathways.

The figure illustrates the 45S pre-rRNA and cleavage sites in the ETS and ITS sequences. In the current models, the 5′ETS of 45S pre-rRNAs is trimmed by XRN2 before being cleaved at the P site by a nucleolin-U3 snoRNP complex. The 3′ETS is cleaved by the RNase III endonuclease, RTL2. Structural rRNAs are methylated by C/D snoRNP (2-O-ribose methylation) or pseudouridylated by H/ACA snoRNP complexes. The resulting 35S(P) pre-rRNA follows either pathway 1, 5′ETS-first (minor), pathway 2, ITS1-first (major), or pathway 3, ITS2-first. Specific processing events in pathway 1, 2, or 3 are shown in the boxes. The characterized protein factors required for RNA modification (C/D and H/ACA snoRNP and TRL), endonucleolytic cleavages (nucleolin-U3 snoRNP, RTL2, and NOB1), or exonucleolytic cleavages (XRN2, AtRRP6L2, and AtRRP41) are indicated.