To identify new genes involved in 3′-end
formation of mRNAs in Saccharomyces cerevisiae,
we carried out a screen for synthetic lethal mutants with
the conditional poly(A) polymerase allele, pap1-7.
Five independent temperature-sensitive mutations called
lcp1 to lcp5 (for lethal with
conditional pap1 allele) were
isolated. Here, we describe the characterization of the
essential gene LCP5 which codes for a protein
with a calculated molecular mass of 40.8 kD. Unexpectedly,
we found that mutations in LCP5 caused defects
in pre-ribosomal RNA (pre-rRNA) processing, whereas mRNA
3′-end formation in vitro was comparable to wild-type.
Early cleavage steps (denoted A0 to A2)
that lead to the production of mature 18S rRNA were impaired.
In vivo depletion of Lcp5p also inhibited pre-rRNA processing.
As a consequence, mutant and depleted cells showed decreased
levels of polysomes compared to wild-type cells. Indirect
immunofluorescence indicated a predominant localization
of Lcp5p in the nucleolus. In addition, antibodies directed
against Lcp5p specifically immunoprecipitated the yeast
U3 snoRNA snR17, suggesting that the protein is directly
involved in pre-rRNA processing.