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 t he processing of pre-rRNA at site A(3) in internal transcribed spacer 1. Although RNase MRP has been analysed functionally, the structure an d composition of the particle are not well characterized, A genetic sc reen for mutants which are synthetically lethal (sl) with a temperatur e-sensitive (ts) mutation in the RNA component of RNase MRP (rrp2-1) i dentified an essential gene, POP3, which encodes a basic protein of 22 .6 kDa predicted molecular weight. Over-expression of Pop3p fully supp resses the ts growth phenotype of the rrp2-1 allele at 34 degrees C an d gives partial suppression at 37 degrees C, Depletion of Pop3p in viv o results in a phenotype characteristic of the loss of RNase MRP activ ity; A(3) 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 A(2), Pop3p depletion also inhibits pre-tRNA processing; tRNA primary transcripts accumulate, as well as spliced but 5'- and 3'-unprocessed pre-tRNAs, The Pop3p deple tion phenotype resembles those previously described for mutations in c omponents of RNase MRP and RNase P (rrp2-1, rpr1-1 and pop1-1). Immuno precipitation of epitope-tagged Pop3p co-precipitates the RNA componen ts of both RNase MRP and RNase P. Pop3p is, therefore, a common compon ent 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 f unction in eukaryotic cells.