ABSTRACT
Prp43p
is a putative helicase of the DEAH family which is required for the
release of the lariat intron from the spliceosome. Prp43p could also
play a role in ribosome synthesis, since it accumulates in the
nucleolus. Consistent with this hypothesis, we find that depletion of
Prp43p leads to accumulation of 35S pre-rRNA and strongly reduces
levels of all downstream pre-rRNA processing intermediates. As a
result, the steady-state levels of mature rRNAs are greatly diminished
following Prp43p depletion. We present data arguing that such effects
are unlikely to be solely due to splicing defects. Moreover, we
demonstrate by a combination of a comprehensive two-hybrid screen,
tandem-affinity purification followed by mass spectrometry, and
Northern analyses that Prp43p is associated with 90S, pre-60S, and
pre-40S ribosomal particles. Prp43p seems preferentially associated
with Pfa1p, a novel specific component of pre-40S ribosomal particles.
In addition, Prp43p interacts with components of the RNA polymerase I
(Pol I) transcription machinery and with mature 18S and 25S rRNAs.
Hence, Prp43p might be delivered to nascent 90S ribosomal particles
during pre-rRNA transcription and remain associated with preribosomal
particles until their final maturation steps in the cytoplasm. Our data
also suggest that the ATPase activity of Prp43p is required for early
steps of pre-rRNA processing and normal accumulation of mature
rRNAs.
High levels of TopBP1 induce ATR-dependent shut-down of rRNA transcription and nucleolar segregation
