Synthesis and assembly of the box C + D small nucleolar RNPs

Two core small nucleolar RNP (snoRNP) proteins, Nop1p (fibrillarin in verte brates) and Nop58p (also known as Nop5p) have previously been reported to b e specifically associated with the box C+D class of small nucleolar RNAs (s noRNAs). Here we report that Nop56p, a protein related in sequence to Nop58 p, is a bona fide box C+D snoRNP component; all tested box C+D snoRNAs were coprecipitated with protein A-tagged Nop56p. Analysis of in vivo snoRNP as sembly indicated that Nop56p was stably associated with the snoRNAs only in the presence of Nop1p. In contrast, Nop58p and Nop1p associate independent ly with the snoRNAs. Genetic depletion of Nop56p resulted in inhibition of early pre-rRNA processing events at sites A(0), A(1), and A(2) and mild dep letion of 18S rRNA. However, Nop56p depletion did not lead to codepletion o f the box C+D snoRNAs. This is in contrast to Nop58p, which was required fo r the accumulation of all tested box C + D snoRNAs. Unexpectedly, we found that Nop1p was specifically required for the synthesis and accumulation of box C+D snoRNAs processed from pre-mRNA introns and polycistronic transcrip ts.