Interdependence in the processing of ribosomal RNAs in Schizosaccharomyces pombe

Eukaryotic rRNAs are produced by cleavage of a large 35 to 45 S pre-rRNA transcript which initially must be fully transcribed and assembled into an 80 to 90 S nucleolar ribonucleoprotein particle. Despite this need for a completed transcript, several investigations have reported a split processing scheme for independent maturation of the large and small subunit rRNAs. Here, an efficiently expressed rDNA plasmid was used to quantitatively analyze the effects of mutations in the internal transcribed spacer (ITS) region in the yeast, Schizosaccharomyces pombe. The results show that substitution of ITS regions inhibits the processing of distant external transcribed spacers (ETS) and that deletion of the ITS2 spacer not only prevents the maturation of the large subunit, but severely affects maturation of the small subunit rRNA. This indicates that the processing mechanisms are not fully split and, when taken together with other evidence of interdependences in rRNA maturation, the results suggest that the interdependences act as a quality control mechanism to help ensure that only functional rRNA is incorporated into ribosomes.