The eukaryotic ribosomal RNAs (18S, 5.8S and 25–28S) are transcribed as a single precursor molecule and processed into mature species by multiple cleavage events. U3 snoRNP, the most abundant of the small nucleolar ribonucleoprotein particle, initiates the early processing events and is absolutely required for the formation of 18S rRNA. The functional U3 snoRNP contains one U3 snoRNA molecule associated with several proteins. Modification exclusion study suggests that the Box-A in the 5′ domain and the Box-B/C in the 3′ of the U3 snoRNA domain form predominant protein binding sites. Several mutations in the U3 snoRNA, spanning both domains, were introduced to probe the structural and functional aspects of U3 snoRNP in S. pombe. The in vitro gel retardation analysis indicated no detectable effect of mutations on the ability of U3 snoRNA to interact with proteins. However, the mutant U3 snoRNA species were synthesized or accumulated inefficiently, and degraded rapidly when expressed in vivo. Together, the results suggest that mutations in the U3 snoRNA did not completely abolish the protein-RNA interactions but resulted into functionally impaired ribonucleoprotein particle.
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