ACCURATE PROCESSING OF A EUKARYOTIC PRECURSOR RIBOSOMAL-RNA BY RIBONUCLEASE MRP IN-VITRO

Very few of the enzymes required for eukaryotic precursor ribosomal RN A (pre-rRNA) processing have been identified, Ribonuclease (RNase) MRP was characterized as a nuclease that cleaves mitochondrial replicatio n primers, but it is predominantly nucleolar, Previous genetic evidenc e revealed that this ribonucleoprotein is required, directly or indire ctly, for cleavage of the yeast pre-rRNA in vivo at site A(3). Here, a n in vitro processing system that accurately reproduces this cleavage is described. Biochemical purification and the use of extracts deplete d of the MRP RNA demonstrate that endonucleolytic cleavage of the pre- rRNA is directly mediated by RNase MRP, This establishes a role for RN ase MRP in the nucleolus.