J Biol Chem, Vol. 275, Issue 2, 1030-1034, January 14, 2000

Purification and Characterization of the tRNA-processing Enzyme RNase BN^*

Colleen Callahan, Doris Neri-Cortes^§¶, and Murray P. Deutscher

From the  Department of Biochemistry and Molecular Biology, University of Miami School of Medicine, Miami, Florida 33101-6129 and the ^§ Department of Biochemistry, University of Connecticut Health Center, Farmington, Connecticut 06032

RNase BN, a tRNA-processing enzyme previously shown to be required for the 3'-maturation of certain bacteriophage T4-encoded tRNAs, was overexpressed and purified to near homogeneity from Escherichia coli. The purified enzyme, which is free of nucleic acid, is an ₂-dimer with a molecular mass of ~65 kDa. RNase BN displays a number of unusual catalytic properties compared with the other exoribonucleases of E. coli. The enzyme is most active at pH 6.5 in the presence of Co²⁺ and high concentrations of monovalent salts. It is highly specific for tRNA substrates containing an incorrect residue within the universal 3'-CCA sequence. Thus, tRNA-CU and tRNA-CA are effective substrates, whereas intact tRNA-CCA, elongated tRNA-CCA-Cn, phosphodiesterase-treated tRNA, and the closely related tRNA-CC are essentially inactive as substrates. RNA or DNA oligonucleotides also are not substrates. These data indicate that RNase BN has an extremely narrow substrate specificity. However, since tRNA molecules with incorrect residues within the -CCA sequence are not normally produced in E. coli, the role of RNase BN in uninfected cells remains to be determined.