THE POP1 GENE ENCODES PROTEIN-COMPONENT COMMON TO THE RNASE-MRP AND RNASE-P RIBONUCLEOPROTEINS

Two forms of the yeast 5.88 rRNA are generated from a large precursor by distinct processing pathways. Cleavage at site A3 is required for s ynthesis of the major, short form, designated 5.8S(S), but not for syn thesis of the long form, 5.8S(L). To identify components required for A3 cleavage, a bank of temperature-sensitive lethal mutants was screen ed for those with a reduced ratio of 5.8S(S):5.8S(L). The pop1-1 mutat ion (for processing of precursor RNAs) shows this phenotype and also i nhibits A3 cleavage. The pre-rRNA processing defect of pop1-1 strains is similar to that reported for mutations in the RNA component of RNas e MRP; we show that a mutation in the RNase MRP RNA also inhibits clea vage at site A3. This is the first site shown to require RNase MRP for cleavage in vivo. The pop1-1 mutation also leads to a block in the pr ocessing of pre-tRNA that is identical to that reported for mutations in the RNA component of RNase P. The RNA components of both RNase MRP and RNase P are underaccumulated in pop1-1 strains at the nonpermissiv e temperature, and immunoprecipitation demonstrates that POP1p is a co mponent of both ribonucleoproteins. The POP1 gene encodes a protein wi th a predicted molecular mass of 100.5 kD and is essential for viabili ty. POP1p is the first protein component of the nuclear RNase P or RNa se MRP for which the gene has been cloned.