SYNTHETIC LETHAL INTERACTIONS WITH CONDITIONAL POLY(A) POLYMERASE ALLELES IDENTIFY LCP5, A GENE INVOLVED IN 18S RIBOSOMAL-RNA MATURATION

To identify new genes involved in 3'-end formation of mRNAs in Sacchar omyces cerevisiae, we carried out a screen for synthetic lethal mutant s with the conditional poly(A) polymerase allele, pap1-7. Five indepen dent temperature-sensitive mutations called Icp1 to Icp5(for lethal wi th conditional pap1 allele) were isolated. Here, we describe the chara cterization of the essential gene LCP5 which codes for a protein with a calculated molecular mass of 40.8 kD. Unexpectedly, we found that mu tations in LCP5 caused defects in pre-ribosomal RNA (pre-rRNA) process ing, whereas mRNA 3'-end formation in vitro was comparable to wild-typ e. Early cleavage steps (denoted Ao to A,) that lead to the production of mature 18S rRNA were impaired. In vivo depletion of Lcp5p also inh ibited pre-rRNA processing. As a consequence, mutant and depleted cell s showed decreased levels of polysomes compared to wild-type cells. In direct immunofluorescence indicated a predominant localization of Lcp5 p in the nucleolus. In addition, antibodies directed against Lcp5p spe cifically immunoprecipitated the yeast U3 snoRNA snR17, suggesting tha t the protein is directly involved in pre-rRNA processing.