Abstract
The structural requirements of E. coli formylmethionine tRNA for aminoacylation have been examined by chemical modification of the tRNA, followed by separation of the modified molecules into active and inactive components. Photooxidation of tRNA(fMet) at 50 degrees in the presence of methylene blue results in modification of two guanosine (G) residues in the acceptor stem, at positions no. 2 and no. 71 from the 5'-phosphate terminus. Both of these modifications are present in inactive molecules, but only the G residue at position no. 2 is modified in the acceptor stem of active molecules. Loss of methionine acceptance occurs with first-order kinetics, indicating that inactivation by modification of G residue no. 71 is independent of any other modifications taking place under these conditions. The presence of a modified G residue at position no. 2 in the acceptor stem of active photooxidized molecules shows that disruption of normal base-pairing in this region is not sufficient to inactivate tRNA(fMet). These data indicate that the inactivating modification at position no. 71 is lethal due to a specific alteration in the nucleotide base, rather than simply as a result of breaking a hydrogen-bonded base pair in the acceptor stem.