Eukaryotic nucleoli contain a large family of box
C+D small nucleolar RNA (snoRNA) species, all of which
are associated with a common protein Nop1p/fibrillarin.
Nop58p was identified in a screen for synthetic lethality
with Nop1p and shown to be an essential nucleolar protein.
Here we report that a Protein A-tagged version of Nop58p
coprecipitates all tested box C+D snoRNAs and that genetic
depletion of Nop58p leads to the loss of all tested box
C+D snoRNAs. The box H+ACA class of snoRNAs are not coprecipitated
with Nop58p, and are not codepleted. The yeast box C+D
snoRNAs include two species, U3 and U14, that are required
for the early cleavages in pre-rRNA processing. Consistent
with this, Nop58p depletion leads to a strong inhibition
of pre-rRNA processing and 18S rRNA synthesis. Unexpectedly,
depletion of Nop58p leads to the accumulation of 3′
extended forms of U3 and U24, showing that the protein
is also involved in snoRNA synthesis. Nop58p is the second
common component of the box C+D snoRNPs to be identified
and the first to be shown to be required for the stability
and for the synthesis of these snoRNAs.