A model for the involvement of the small nucleolar RNA (U3) in processing eukaryotic ribosomal RNA

The nucleotide sequence of chick pre-rRNA between 5.8S and 28S rRNAs is 85% G + C and has the potential to form many different secondary structures. A model is presented in which a small nucleolar RNA, U3, and its associated proteins act as an RNA isomerase to position the pre-rRNA for processing. Cleavage could be performed either by a nuclease present in the U3RNP or by a ribonuclease directed to the proper form of the pre-rRNA.