Fig. 3.

Maturation pathways of the 35S pre-rRNA transcript in Saccharomyces cerevisiae (A) and the 47S pre-RNA transcript in Homo sapiens (C). Three of the four rRNAs (18S, 5.8S, and 25S (in yeast)/28S (in humans)) are synthesized by Pol I as a single long transcript. The coding sequences of mature rRNAs are flanked by 5’- and 3’-ETS, ITS1, and ITS2 non-coding spacers. The schematic shows the relative position of known and predicted cleavage sites. (B) Processing of pre-rRNA in budding yeast. (D) A simplified schematic of human pre-rRNA processing. The primary transcript, 47S pre-rRNA, is initially cleaved at both ends at sites 01 and 02 to form the 45S precursor that is processed via two alternative pathways [51]. “>” (e.g., C2>C1’>C1) denotes consecutive shortening of the appropriate 3’- or 5’-ends of the pre-rRNA by nucleases