THE ROLE OF THE 3'-EXTERNAL TRANSCRIBED SPACER IN YEAST PRE-RIBOSOMAL-RNA PROCESSING

We have undertaken a deletion analysis of the 3' external transcribed spacer (3' ETS) in the pre-rRNA of Saccharomyces cerevisiae. A stem lo op structure immediately 3' to the 25 S rRNA region is necessary and s ufficient for processing of the 3' ETS. This is believed to be by cotr anscriptional cleavage by Rntlp, the yeast homologue of RNase III. Ln addition, this stem-loop is required for cleavage of site A(3) by RNas e MRP and for processing at site B-IL in the 3' region of ITS1. Proces sing at an upstream site in ITS1, site A(2), and at sites in the 5' ex ternal transcribed spacer are not affected, even by complete deletion of the 3' ETS. We conclude that processing in the 3' ETS and in ITS1 i s coupled. This would constitute a quality control that prevents synth esis of the 5.8 S rRNA and 5' end maturation of the 25 S rRNA in trans cripts which are incomplete due to premature transcription termination . (C) 1998 Academic Press Limited.