Functional group recognition at the aminoacylation and editing sites of E. coli valyl-tRNA synthetase

To correct misactivation and misacylation errors, Escherichia coli valyl-tRNA synthetase (ValRS) catalyzes a tRNA Val -dependent editing reaction at a site distinct from its aminoacylation site. Here we examined the effects of replacing the conserved 3′-adenosine of tRNA Val with nucleoside analogs, to identify structural elements of the 3′-terminal nucleoside necessary for tRNA function at the aminoacylation and editing sites of ValRS. The results show that the exocyclic amino group (N6) is not essential: purine riboside-substituted tRNA Val is active in aminoacylation and in stimulating editing. Presence of an O6 substituent (guanosine, inosine, xanthosine) interferes with aminoacylation as well as posttransfer and total editing (pre- plus posttransfer editing). Because ValRS does not recognize substituents at the 6-position, these results suggest that an unprotonated N1, capable of acting as an H-bond acceptor, is an essential determinant for both the aminoacylation and editing reactions. Substituents at the 2-position of the purine ring, either a 2-amino group (2-aminopurine, 2,6-diaminopurine, guanosine, and 7-deazaguanosine) or a 2-keto group (xanthosine, isoguanosine), strongly inhibit both aminoacylation and editing. Although aminoacylation by ValRS is at the 2′-OH, substitution of the 3′-terminal adenosine of tRNA Val with 3′-deoxyadenosine reduces the efficiency of valine acceptance and of posttransfer editing, demonstrating that the 3′-terminal hydroxyl group contributes to tRNA recognition at both the aminoacylation and editing sites. Our results show a strong correlation between the amino acid accepting activity of tRNA and its ability to stimulate editing, suggesting misacylated tRNA is a transient intermediate in the editing reaction, and editing by ValRS requires a posttransfer step.