Figure 1.

Simplified scheme of the pre-rRNA processing pathway in Saccharomyces cerevisiae. (A) Initial 35S pre-rRNA precursor with detailed cleavage sites as well as probes used for rRNA detection in northern blot analysis. (B) Endonucleolytic and exonucleolytic cleavages leading to the production of mature 18S, 5.8S and 25S rRNAs. Inhibition of A₀, A₁ and A₂ cleavages (A2 pathway) leads to the non-productive A₃ pathway. Resulting 23S pre-rRNA containing particles accumulate and/or are degraded. Dark and light gray boxes schematically represent 90S and 43S/66S pre-ribosomes, respectively. Mature ribosomal subunits are represented in violet.