In order to identify amino acids in Thermus thermophilus elongation factor Tu which contribute to its specificity for different tRNAs, the binding affinities of 20 point mutations were compared to that of wild type protein using four tRNAs of differing affinities. The observed specificity for tRNA is the result of the varying contributions of five amino acids which make contacts with the T-stem and the 3′ terminus of tRNA. For three of the amino acids the test tRNAs differ in sequence at the site of contact, presumably explaining the specificity. However, the remaining two amino acids contact tRNA at conserved positions, suggesting that more global structural or dynamic properties of the free tRNA contribute to specificity.
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