Pop3p is essential for the activity of the RNase MRP and RNase P ribonucleoproteins in vivo

RNase MRP is a ribonucleoprotein (RNP) particle which is involved in the processing of pre‐rRNA at site A3 in internal transcribed spacer 1. Although RNase MRP has been analysed functionally, the structure and composition of the particle are not well characterized. A genetic screen for mutants which are synthetically lethal (sl) with a temperature‐sensitive (ts) mutation in the RNA component of RNase MRP (rrp2‐1) identified an essential gene, POP3, which encodes a basic protein of 22.6 kDa predicted molecular weight. Overexpression of Pop3p fully suppresses the ts growth phenotype of the rrp2‐1 allele at 34°C and gives partial suppression at 37°C. Depletion of Pop3p in vivo results in a phenotype characteristic of the loss of RNase MRP activity; A3 cleavage is inhibited, leading to under‐accumulation of the short form of the 5.8S rRNA (5.8SS) and formation of an aberrant 5.8S rRNA precursor which is 5′‐extended to site A2. Pop3p depletion also inhibits pre‐tRNA processing; tRNA primary transcripts accumulate, as well as spliced but 5′‐ and 3′‐unprocessed pre‐tRNAs. The Pop3p depletion phenotype resembles those previously described for mutations in components of RNase MRP and RNase P (rrp2‐1, rpr1‐1 and pop1‐1). Immunoprecipitation of epitope‐tagged Pop3p co‐precipitates the RNA components of both RNase MRP and RNase P. Pop3p is, therefore, a common component of both RNPs and is required for their enzymatic functions in vivo. The ubiquitous RNase P RNP, which has a single protein component in Bacteria and Archaea, requires at least two protein subunits for its function in eukaryotic cells.