FIG. 1.

(A) Pre-rRNA processing in the yeast S. cerevisiae. The nascent 35S transcript is cleaved in three U3 snoRNP-dependent reactions at A0, A1, and A2 to separate the precursors to the SSU (18S) rRNA from those to the LSU (5.8S and 25S) rRNAs. (B) Pre-rRNA processing pathway in yeast depleted of the U3 snoRNA. Genetic depletion of the U3 snoRNA causes an apparent increase in the levels of the nascent 35S precursor and accumulation of the 23S precursor resulting from lack of cleavage at A0, A1, and A2. As a result, decreased levels of the 20S and 18S rRNAs occur. Processing of the LSU pre-rRNA occurs via the 27S precursor (18).