| Abstract
| - Three previously described mutant Escherichiacoli glutaminyl-tRNA synthetase (GlnRS)proteins that incorrectly aminoacylate the amber suppressor derivedfrom tRNATyr (supF) with glutaminewere cocrystallized with wild-type tRNAGln and theirstructures determined. In two of the mutant enzymesstudied, Asp235, which contacts base pair G3-C70 in the acceptor stem,has been changed to asparaginein GlnRS7 and to glycine in GlnRS10. These mutations result inchanged interactions between Asn235of GlnRS7 and G3-C70 of the tRNA and an altered water structure betweenGly235 of GlnRS10 andbase pair G3-C70. These structures suggest how the mutant enzymescan show only small changes intheir ability to aminoacylate wild-type cognate tRNA on the one handand yet show a lack of discriminationagainst a noncognate U3-A70 base pair on the other. In contrast,the change of Ile129 to Thr in GlnRS15causes virtually no change in the structure of the complex, and theexplanation for its ability to misacylatesupF is unclear.
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