Prevention of mis-aminoacylation of a dual-specificity aminoacyl-tRNA synthetase

Accurate aminoacylation of tRNAs by aminoacyl-tRNA synthetase is essential for the fidelity of protein synthesis. For Methanococcus jannaschii tRNAPro, accuracy is difficult because the cognate prolyl-tRNA synthetase also recognizes and aminoacylates tRNACys with cysteine. We show here that the unmodified transcript of M. jannaschii tRNAPro is indeed mis-acylated with cysteine. However, the origin of mis-charging is not at the anticodon or acceptor stem, the two hotspots for tRNAPro and tRNACys identity determinants. Instead, replacement of the D loop in the tRNA core with that of tRNACys suppresses mis-charging with cysteine without compromising the activity of aminoacylation with proline. The reduced level of cysteine activity of the chimera is not due an editing response of the synthetase and is consistent with a relaxed sensitivity of the tRNA to the analog thiaproline in aminoacylation with cysteine. We suggest that mis-acylation is not due to the presence of cysteine determinants, but to a mis-placed 3′ end into the cysteine catalytic site that activates and transfers cysteine to the tRNA. Prevention of mis-placement by alteration of the core structure or by nucleotide modifications in the tRNA illustrates a novel strategy of the dual-specificity synthetase.