Chemical Evidence for a Codon‐Induced Allosteric Change in tRNALys Involving the 7‐Methylguanosine Residue 46

Abstract

[³²P]tRNA^LyS, from Escherichia coli, was modified with kethoxal, in the presence and absence of the oligonucleotide codon (A)₄. The presence of the codon resulted in a faster modification rate of the tRNA at three guanine sites which were identified by a diagonal fingerprint method. A large increase in the modification rate occurred at the 7-methylguanosine residue 46 (m⁷G-46) in the presence of the codon; weakly enhanced modification was observed at G-15 and G-57. It is concluded that the formation of a codon-anticodon complex induces, primarily, a confor-mational change involving disruption of the m⁷G-46 from the m⁷G-46 · G-22 · C-13 base triple. Subsequently, the guanines of G-15 and G-57, in the D and T loops, respectively, become slightly more reactive, suggesting a weak tendency for these two interacting arms to unfold. The results are interpreted in terms of an equilibrium between two main conformers and a third minor one; the possible significance of these conformers, in protein biosynthesis, is considered.