SINGLE SEQUENCE OF A HELIX-LOOP PEPTIDE CONFERS FUNCTIONAL ANTICODON RECOGNITION ON 2 TRANSFER-RNA SYNTHETASES

The specific aminoacylation of RNA oligonucleotides whose sequences ar e based on the acceptor stems of tRNAs can be viewed as an operational RNA code for amino acids that may be related to the development of th e genetic code. Many synthetases also have direct interactions with tR NA anticodon triplets and, in some cases, these interactions are thoug ht to be essential for aminoacylation specificity. In these instances, an unresolved question is whether interactions with parts of the tRNA outside of the anticodon are sufficient for decoding genetic informat ion. Escherichia coil isoleucyl- and methionyl-tRNA synthetases are cl osely related enzymes that interact with their respective anticodons, We used binary combinatorial mutagenesis of a 10 amino acid anticodon binding peptide in these two enzymes to identify composite sequences t hat would confer function to both enzymes despite their recognizing di fferent anticodons. A single peptide was found that confers function t o both enzymes in vivo and in vitro. Thus, even in enzymes where antic odon interactions are normally important for distinguishing one tRNA f rom another, these interactions can be 'neutralized' without losing sp ecificity of aminoacylation. We suggest that acceptor helix interactio ns may play a role in providing the needed specificity.