Structural basis for substrate binding, cleavage and allostery in the tRNA maturase RNase Z

Transfer RNAs ( tRNAs) are synthesized as part of longer primary transcripts that require processing of both their 30 and 50 extremities in every living organism known. The 50 side is processed ( matured) by the ubiquitously conserved endonucleolytic ribozyme, RNase P-1, whereas removal of the 30 tails can be either exonucleolytic(2,3) or endonucleolytic(4). The endonucleolytic pathway is catalysed by an enzyme known as RNase Z, or 3' tRNase(5,6). RNase Z cleaves precursor tRNAs immediately after the discriminator base ( the unpaired nucleotide 30 to the last base pair of the acceptor stem, used as an identity determinant by many aminoacyl- tRNA synthetases) in most cases(6 - 8), yielding a tRNA primed for addition of the CCA motif by nucleotidyl transferase. Here we report the crystal structure of Bacillus subtilis RNase Z at 2.1 Angstrom resolution, and propose a mechanism for tRNA recognition and cleavage. The structure explains the allosteric properties of the enzyme, and also sheds light on the mechanisms of inhibition by the CCA motif and long 50 extensions. Finally, it highlights the extraordinary adaptability of the metallo- hydrolase domain of the beta- lactamase family for the hydrolysis of covalent bonds.