TYR-426 OF THE ESCHERICHIA-COLI ASPARAGINYL-TRANSFER RNA-SYNTHETASE, AN AMINO-ACID IN A C-TERMINAL CONSERVED MOTIF, IS INVOLVED IN ATP BINDING

Sequence comparisons of the E. coli asparaginyl-tRNA synthetase (NRSEC) with aminoacyl-tRNA synthetase sequences of class II enzymes show significant homologies with aspartyl- and lysyl-tRNA synthetases. Three conserved regions were found, one of which is located in the C-terminal part of the NRSEC sequence. Site-directed mutagenesis was performed in this conserved region. A single point mutation Tyr-426-->Ser results in a 15-fold increase in the K(m) for ATP, while all the other kinetic parameters remain unchanged. The replacement of this Tyr-426 by a Phe does not affect the kinetic behaviour of the enzyme. These data indicate that Tyr-426 is part of the ATP binding site.