5'ETS RIBOSOMAL-RNA PROCESSING FACILITATED BY 4 SMALL RNAS - U14, E3,U17, AND U3

The nucleolus, the compartment in which the large ribosomal RNA precur sor (pre-rRNA) is synthesized, processed through a series of nucleolyt ic cleavages and modifications into the mature 18S, 5.8S, and 28S rRNA s, and assembled with proteins to form ribosomal subunits, also contai ns many small nucleolar RNAs (snoRNAs). We present evidence that the f irst processing event in mouse rRNA maturation, cleavage within the 5' external transcribed spacer, is facilitated by at least four snoRNAs: U14, U17(E1), and E3, as well as U3. These snoRNAs do not augment thi s processing by directing 2'-O-methylation of the pre-rRNA. A macromol ecular complex in which this 5'ETS processing occurs may then function in the processing of 18S rRNA.