Apparent involvement of ribonuclease D in the 3' processing of tRNA precursors.

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Abstract
Escherichia coli RNase D and RNase II were purified to homogeneity and compared for their ability to remove extra nucleotides following the -C-C-A sequence in tRNA precursors. RNase D and RNase II are single-chain proteins with MW of 38,000 and 78,000, respectively. Both enzymes require a divalent cation for activity on tRNA precursors, but, in addition, RNase II is stimulated by monovalent cations. RNase D specifically removes mononucleotide residues from a mixture of tRNA precursors to generate amino acid acceptor activity is recovered. RNase D action on the E. coli tRNATyr precursor is limited, whereas RNase II causes extensive degradation. In contrast to the processive mode of hydrolysis by RNase II, RNase D removes nucleotides randomly and slows down greatly at the -C-C-A sequence, thereby allowing the tRNA to be aminocylated and protected from further degradation. RNase D is the 3''-processing nuclease in vivo, and RNase II is a nonspecific degradative enzyme. The importance of RNA conformation for direct processing is discussed.