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.