RNASE-III CLEAVES EUKARYOTIC PRERIBOSOMAL RNA AT A U3 SNORNP-DEPENDENT SITE

A yeast gene homologous to bacterial RNase III (RNT1) encodes a double -strand-specific endoribonuclease essential for ribosome synthesis. Tw o rRNA processing events are blocked in cells temperature sensitive fo r RNT1: cleavage at the snoRNA-dependent A0 site in the 5' ETS and cle avage in the 3' ETS. Recombinant RNT1 protein accurately cleaves a syn thetic 5' ETS RNA at the AO site in vitro, in the absence of snoRNA or other factors. A synthetic 3' ETS substrate is specifically cleaved a t a site 21 nt downstream of the 3' end of 28S rRNA. These observation s show that a protein endonuclease collaborates with snoRNAs in eukary otic rRNA processing and exclude a catalytic role for snoRNAs at certa in pre-rRNA cleavage sites.