The genetic depletion of yeast Rrp5p results in
a synthesis defect of both 18S and 5.8S ribosomal RNAs
(Venema J, Tollervey D. 1996. EMBO J 15:5701–5714).
We have isolated the RRP5 gene in a genetic approach
aimed to select for yeast factors interfering with protein
import into mitochondria. We describe here a striking feature
of Rrp5p amino acid sequence, namely the presence of twelve
putative S1 RNA-binding motifs and seven tetratricopeptide
repeats (TPR) motifs. We have constructed two conditional
temperature-sensitive alleles of RRP5 gene and
analyzed them for associated rRNA-processing defects. First,
a functional “bipartite gene” was generated
revealing that the S1 and TPR parts of the protein can
act independently of each other. We also generated a two
amino acid deletion in TPR unit 1 (rrp5Δ6
allele). The two mutant forms of Rrp5p were shown to cause
a defect in 18S rRNA synthesis with no detectable effects
on 5.8S rRNA production. However, the rRNA processing pathway
was differently affected in each case. Interestingly, the
ROK1 gene which, like RRP5, was previously
isolated in a screen for synthetic lethal mutations with
snR10 deletion, was here identified as a high copy suppressor
of the rrp5Δ6 temperature-sensitive allele.
ROK1 also acts as a low copy suppressor but cannot
bypass the cellular requirement for RRP5. Furthermore,
we show that suppression by the Rok1p putative RNA helicase
rescues the 18S rRNA synthesis defect caused by the rrp5Δ6
mutation.