A tRNA identity switch mediated by the binding interaction between a tRNA anticodon and the accessory domain of a class II aminoacyl-tRNA synthetase

Identity elements in tRNAs and the intracellular balance of tRNAs allow accurate selection of tRNAs by aminoacyl-tRNA synthetases. The histidyl-tRNA from Escherichia coli is distinguished by a unique G-1 . C73 base pair that upon exchange with other nucleotides leads to a marked decrease in the rate of aminoacylation in vitro. G-1 . C73 is also a major identity element for histidine acceptance, such that the substitution of C73 brings about mischarging by glycyl-, glutaminyl-, and leucyl-tRNA synthetases. These identity conversions mediated by the G-1 . C73 base pair were exploited to isolate secondary site revertants in the histidyl-tRNA synthetase from E. coli which restore histidine identity to a histidyl-tRNA suppressor carrying U73. The revertant substitutions confer a 3-4-fold reduction in the Michaelis constant for tRNAs carrying the amber-suppressing anticodon and map to the C-terminal domain of HisRS and its interface with the catalytic core. These findings demonstrate that the histidine tRNA anticodon plays a significant role in tRNA selection in vivo and that the C-terminal domain of HisRS is in large part responsible for recognizing this trinucleotide. The kinetic parameters determined also show a small degree of anticooperativity (Delta Delta G = -1.24 kcal/mol) between recognition of the discriminator base and the anticodon, suggesting that the two helical domains of the tRNA are not recognized independently. We propose that these effects substantially account for the ability of small changes in tRNA binding far removed from the site of a major determinant to bring about a complete conversion of tRNA identity.