FIG. 3.

Schematic diagram of pre-rRNA processing in the yeast S. cerevisiae. (A) Organization and processing sites of 35S pre-rRNA. The 35S precursor contains the sequences for the mature 18S, 25S, and 5.8S rRNAs, which are separated by the internal transcribed spacers ITS1 and ITS2 and two external transcribed spacers, 5′ETS and 3′ETS, at the 5′ and 3′ ends, respectively. The oligonucleotides corresponding to different regions of 35S rRNA that were used as probes in Northern analysis are indicated by numbers 1 through 9. (B) Pre-rRNA processing steps and different processing intermediates. The major cleavage sites, endonucleolytic and exonucleolytic processing steps, processing intermediates, and pathways leading to mature rRNA are indicated (adapted from reference 11).