Figure 2.

Eukaryotic rRNA processing in yeast. (A) Schematic depiction of the yeast rDNA locus on chromosome XII comprising 150 tandem repeats. Each repeat contains the sequence for the 5S rRNA which is transcribed by Pol III and the 35S pre-rRNA (comprising the 18S, 5.8S and 25S rRNA) which is synthesized by Pol I with a transcription rate of about 40 nucleotides per second in the opposite direction [20]. (B) Schematic overview of the 35S pre-rRNA processing cascade. During the exponential growth phase 70% of the nascent transcripts are co-transcriptionally cleaved at the A₂ site (NTC: nascent transcript cleavage), while 30% are cleaved post-transcriptionally (RTC: released transcript cleavage) [20]. (C) Lifetimes (brackets) and processing times of pre-rRNAs in seconds (s). Koš and Tollervey (2010) determined the lifetimes and processing times of pre-RNAs via metabolic labeling [20]. The processing time of the 35S pre-rRNA to the indicated rRNAs is given in a range, since it is dependent on whether the A₂ cleavage occurs co- or post-transcriptionally.