A variant (LeuRS-A) of Escherichia coli leucyl-tRNA synthetase (LeuRS) carrying a 40-residue duplication in its connective peptide 1 (CP1) has a 3- fold lower specificity for tRNA1/(Leu) than for tRNA2/(Leu), whereas wild- type LeuRS has the same specificity for these two isoacceptors. The replacement of the acceptor stem of tRNA2/(Leu) with tRNA1/(Leu) yields a chimeric tRNA(Leu) for which wild-type LeuRS has the same specificity as it does for the two normal isoacceptors mentioned, but for which LeuRS-A has a reduced specificity similar to that for tRNA1/(Leu), indicating a difference between these two acceptor stems. LeuRS-A is slightly less stable than the native enzyme. Wild-type LeuRS and LeuRS-A have almost same K(d) value for their interaction with tRNA1/(Leu) as determined by fluorescence quenching. No difference was detected between these two proteins by CD and fluorescence spectroscopy. These results show that LeuRS-A can discriminate between the two isoacceptors of tRNA(Leu).
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