This proposal focuses on genetic approaches to the specificity of interactions between aminoacyl tRNA synthetases and tRNAs. The rationale is that the tRNA binding sites on the various synthetases share common architectural features, and that specificity changes can be achieved through manipulation of the tRNA binding sites. Various approaches will be used to isolate mutant aminoacyl tRNA synthetases with changed tRNA specificities. Synthetase genes are inserted into hybrid ColEl plasmid molecules. The plasmids are to be mutagenized and subsequently used to select for synthetase mutants. Synthetases with altered tRNA specificities will be selected as missense suppressors, as well as by other procedures. Thorough biochemical identification and characterization of the altered enzymes is outlined. Together with x-ray crystallographic information and other chemical studies, the isolation and characterization of such mutants will provide unique insight into the mechanism of recognition.