Relaxation of a transfer RNA specificity by removal of modified nucleotides

THE molecular recognition of specific transfer RNAs by the appropriate aminoacyl-tRNA synthetase is an important step in determining the accuracy of translation of the genetic message from nucleic acids into proteins. Recent studies using variant tRNAs with specifie sequence modifications have indicated particular regions that determine their identity 1–8 . Here we consider whether the base modifications commonly found in tRNAs 9 contribute to their identity. Although unmodified tRNA Asp is charged with aspartate as efficiently as the modified native tRNA, it is mischarged with arginine with considerably increased efficiency. Our results indicate that post-transcriptional modification of tRNAs introduces structural 'anti-determinants', restricting the efficiency with which the tRNAs are charged with inappropriate amino acids.