Dual mode recognition of two isoacceptor tRNAs by mammalian mitochondrial seryl-tRNA synthetase

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; tRNA(GCU)(Ser) (for AGY) lacks the entire D arm, whereas tRNA(UGA)(Ser) (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 T PsiC loop of each isoacceptor, and kinetic studies revealed that mt SerRS specifically recognized the T PsiC loop sequence in each isoacceptor. However, in the case of tRNA(UGA)(Ser), T PsiC 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 tRNA(Gln), which has the same T PsiC loop sequence as tRNA(UGA)(Ser), implying that the fidelity of mitochondrial translation is maintained by kinetic discrimination of tRNAs in the network of aminoacyl-tRNA synthetases.