5'-END MATURATION AND RNA EDITING HAVE TO PRECEDE TRANSFER-RNA 3'-PROCESSING IN PLANT-MITOCHONDRIA

We report the characterization and partial purification of potato mito chondrial RNase Z, an endonuclease that generates mature tRNA 3' ends. The enzyme consists of one (or more) protein(s) without RNA subunits. Products of the processing reaction are tRNA molecules with 3' termin al hydroxyl groups and 3' trailers with 5' terminal phosphates. The ma in processing sites are located immediately 3' to the discriminator an d one nucleotide further downstream. This endonucleolytic processing a t and close to the tRNA 3' end in potato mitochondria suggests a highe r similarity to the eukaryotic than to the prokaryotic tRNA 3' process ing pathway. Partial purification and separation of RNase Z from the 5 ' processing activity RNase P allowed us to determine biochemical char acteristics of the enzyme. The activity is stable over broad pH and te mperature ranges, with peak activity at pH 8 and 30 degrees C. Optimal concentrations for MgCl2 and KCl are 5 mM and 30 mM, respectively. Th e potato mitochondrial RNase Z accepts only tRNA precursors with matur e 5' ends. The precursor for tRNA(Phe) requires RNA editing for effici ent processing by RNase Z.