An unexpected, conserved element of the U3 snoRNA is required for Mpp10p association

The U3 small nucleolar ribonucleoprotein (snoRNP) is composed of a small nu cleolar RNA (snoRNA) and at least 10 proteins. The U3 snoRNA base pairs wit h the pre-rRNA to carry out the A0, A1, and A2 processing reactions that le ad to the release of the 18S rRNA from the nascent pre-rRNA transcript. The yeast U3 snoRNA can be divided into a short 5' domain (nt 1-39) and a larg er 3' domain (73 to the 3' end) separated by a stretch of nucleotides calle d the hinge region (nt 40-72). The sequences required for pre-rRNA base pai ring are found in the 5' domain and hinge region whereas the 3' domain is l argely covered with proteins. Mpp10p, one of the protein components unique to the U3 snoRNP, plays a role in processing at the A1 and A2 sites. Becaus e of its critical role in U3 snoRNP function, we determined which sequences in the U3 snoRNA are required for Mpp10p association. Unlike fibrillarin a nd all the previous U3 snoRNP components studied in this manner, sequences in the 3' domain are not sufficient for Mpp10p association. Instead, a cons erved sequence element in the U3 snoRNA hinge region is required, placing M pp10p near the 5' domain that carries out the pre-rRNA base-pairing interac tions in the functional center of the U3 snoRNP.