FIGURE 10-41 Pre-rRNA processing in yeast. Endoribonucleases that make internal cleavages are represented as scissors. Exoribonucleases that digest from one end, either 5′ or 3′, are shown as Pac-Men. Most 2′-O-ribose methylation (CH₃) and generation of pseudouridines (Ψ) in the rRNAs occurs following the initial cleavage at the 3′ end, before the initial cleavage at the 5′ end. Proteins and snoRNPs known to participate in these steps are indicated. See J. Venema and D. Tollervey, 1999, Annu. Rev. Genet. 33:261.