ASSOCIATION OF RNASE MITOCHONDRIAL RNA PROCESSING ENZYME WITH RIBONUCLEASE-P IN HIGHER ORDERED STRUCTURES IN THE NUCLEOLUS - A POSSIBLE COORDINATE ROLE IN RIBOSOME BIOGENESIS

RNase mitochondrial RNA processing enzyme (MRP) is a nucleolar ribonuc leoprotein particle that participates in 5.8S ribosomal RNA maturation in eukaryotes. This enzyme shares a polypeptide and an RNA structural moth with ribonuclease P (RNase P), a nuclear endoribonuclease origin ally described in the nucleus that processes tRNA transcripts to gener ate their mature 5' termini. Both enzymes are also located in mitochon dria. This report further characterizes the relationship between RNase MRP and RNase P. Antisense affinity selection with biotinylated 2'-O- methyl oligoribonucleotides and glycerol gradient fractionation experi ments demonstrated that small subpopulations of RNase MRP and RNase P associate with each other in vivo in a macromolecular complex, possibl y 60-80S preribosomes. This latter notion was supported by fluorescenc e in situ hybridization experiments with antisense oligonucleotides th at localized the RNA components of RNase MRP and RNase P to the nucleo lus and to discrete cytoplasmic structures. These findings suggest tha t small subpopulations of RNase MRP and RNase P are physically associa ted, and that both may function in ribosomal RNA maturation or ribosom e assembly.