The plant tRNA 3 ' processing enzyme has a broad substrate spectrum

To elucidate the minimal substrate for the plant nuclear tRNA 3 ' processin g enzyme, we synthesized a set of tRNA variants, which were subsequently in cubated with the nuclear tRNA 3 ' processing enzyme. Our experiments show t hat the minimal substrate for the nuclear RNase Z consists of the acceptor stem and T arm. The broad substrate spectrum of the nuclear RNase Z raises the possibility that this enzyme might have additional functions in the nuc leus besides tRNA 3 ' processing. incubation of tRNA variants with the plan t mitochondrial enzyme revealed that the organellar counterpart of the nucl ear enzyme has a much narrower substrate spectrum. The mitochondrial RNase Z only tolerates deletion of anticodon and variable arms and only with a dr astic reduction in cleavage efficiency, indicating that the mitochondrial a ctivity can only cleave bona fide tRNA substrates efficiently, Both enzymes prefer precursors containing short 3 ' trailers over extended 3 ' addition al sequences. Determination of cleavage sites showed that the cleavage site is not shifted in any of the tRNA variant precursors.