tRNA-dependent amino acid discrimination by yeast seryl-tRNA synthetase

The ability of aminoacyl-tRNA synthetases to distinguish between similar amino acids is crucial for accurate translation of the genetic code. Saccharomyces cerevisiae seryl-tRNA synthetase (SerRS) employs tRNA-dependent recognition of its cognate amino acid serine [Lenhard, B., Filipic, S., Landeka, I., Skrtic, I., Soll, D. & Weygand-Durasevic, I. ( 1997) J. Biol. Chem. 272, 1136 1141]. Here we show that dimeric SerRS enzyme complexed with ne molecule of tRNA(Ser) is more specific and more efficient in catalyzing seryl-adenylate formation than the apoenzyme alone. Sequence-specific tRNA protein interactions enhance discrimination of the amino acid substrate by yeast SerRS and diminish the misactivation of the structurally similar noncognate threonine. This may proceed via a tRNA-induced conformational change in the enzyme's active site. The 3'-terminal adenosine of tRNA(Ser) is not important in effecting the rearrangement of the serine binding site. Our results do not provide an indication for a readjustment of ATP binding in a tRNA-assisted manner. The stoichiometric analyses of the complexes between the enzyme and tRNA(Ser) revealed that two cognate tRNA molecules can be bound to dimeric SerRS, however, with very different affinities.