Animal mitochondrial translation systems contain two serine tRNAs, corresponding to the codons AGY (Y = U and C) and UCN (N = U, C, A, and G), each possessing an unusual secondary structure; tRNAGCUSer(for AGY) lacks the entire D arm, whereas tRNAUGASer(for UCN) has an unusual cloverleaf configuration. We previously demonstrated that a single bovine mitochondrial seryl-tRNA synthetase (mt SerRS) recognizes these topologically distinct isoacceptors having no common sequence or structure. Recombinant mt SerRS clearly footprinted at the TΨC loop of each isoacceptor, and kinetic studies revealed that mt SerRS specifically recognized the TΨC loop sequence in each isoacceptor. However, in the case of tRNAUGASer, TΨC loop-D loop interaction was further required for recognition, suggesting that mt SerRS recognizes the two substrates by distinct mechanisms. mt SerRS could slightly but significantly misacylate mitochondrial tRNAGln, which has the same TΨC loop sequence as tRNAUGASer, implying that the fidelity of mitochondrial translation is maintained by kinetic discrimination of tRNAs in the network of aminoacyl-tRNA synthetases.
Translation of a Yeast Mitochondrial tRNA Synthetase Initiated at Redundant non-AUG Codons
