The U3 small nucleolar ribonucleoprotein (snoRNP) is
composed of a small nucleolar RNA (snoRNA) and at least
10 proteins. The U3 snoRNA base pairs with the pre-rRNA
to carry out the A0, A1, and A2 processing reactions that
lead 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 larger 3′ domain
(73 to the 3′ end) separated by a stretch of nucleotides
called the hinge region (nt 40–72). The sequences
required for pre-rRNA base pairing are found in the 5′
domain and hinge region whereas the 3′ domain is
largely 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. Because of its critical role in
U3 snoRNP function, we determined which sequences in the
U3 snoRNA are required for Mpp10p association. Unlike fibrillarin
and all the previous U3 snoRNP components studied in this
manner, sequences in the 3′ domain are not sufficient
for Mpp10p association. Instead, a conserved sequence element
in the U3 snoRNA hinge region is required, placing Mpp10p
near the 5′ domain that carries out the pre-rRNA
base-pairing interactions in the functional center of the
U3 snoRNP.