CRYSTAL-STRUCTURES OF 3 MISACYLATING MUTANTS OF ESCHERICHIA-COLI GLUTAMINYL-TRANSFER-RNA SYNTHETASE COMPLEXED WITH TRNA(GLN) AND ATP

Three previously described mutant Escherichia coli glutaminyl-tRNA syn thetase (GlnRS) proteins that incorrectly aminoacylate the amber suppr essor derived from tRNA(Tyr) (supF) With glutamine were cocrystallized with wild-type tRNA(Gln) and their structures determined. In two of t he mutant enzymes studied, Asp235, which contacts base pair G3-C70 in the acceptor stem, has been changed to asparagine in GlnRS7 and to gly cine in GlnRS10. These mutations result in changed interactions betwee n Asn235 of GlnRS7 and G3-C70 of the tRNA and an altered water structu re between Gly235 of GlnRS10 and base pair G3-C70. These structures su ggest how the mutant enzymes can show only small changes in their abil ity to aminoacylate wild-type cognate tRNA on the one hand and yet sho w a lack of discrimination against a noncognate U3-A70 base pair on th e other. In contrast, the change of Ile129 to Thr in GlnRS15 causes vi rtually no change in the structure of the complex, and the explanation for its ability to misacylate supF is unclear.