1. The Fast Track Is Cotranscriptional
Jonathan R. Warner, Hyun-Soo Kim 
Molecular Cell 
Volume 37, Issue 6, Pages (March 2010)
DOI: /j.molcel
Copyright © 2010 Elsevier Inc. Terms and Conditions

2. Figure 1
Alternative Routes for rRNA Processing in Saccharomyces cerevisiae
(A) The complete 35S transcript is processed through a complex pathway employing ∼200 proteins and ∼100 RNA molecules (see Henras et al., 2008, for a thorough review). Critical cleavage sites are indicated. Sites A1 and A2 are thought to be cleaved nearly simultaneously by an as-yet-unidentified enzyme. Site B0 is cleaved at a stem loop by Rnt1. Site B1 is cleaved by the intriguing RNase mitochondrial RNA processing (MRP) that has multiple functions in the cell (see Lu et al., 2010). Not to scale, as indicated by the slashes in 18S and 25S RNAs.
(B) The alternative processing routes proposed by Kos and Tollervey (2010), with 70% of the transcripts cleaved at A2 some 25–30 s (1200 nt) after the RNA Pol I enzyme has passed the A2 site. We speculate that cotranscriptionally cleaved 27SA molecules can begin to recruit RNase MRP while Pol I is completing transcription. Thus they would have a head start over the posttranscriptionally cleaved 27SA molecules. Dots on the transcripts reflect another critical observation of Kos and Tollervey, namely that methylation of 18S and of some 25S rRNAs occurs during transcription, which has substantial implications for the intricate folding of rRNA (see text).
Molecular Cell  , DOI: ( /j.molcel )
Copyright © 2010 Elsevier Inc. Terms and Conditions