Evidence from ultraviolet absorbance measurements for a codon-induced conformational change in lysine tRNA from Escherichia coli.

From experiments with equilibrium dialysis it was concluded earlier that formation of the codon-anticodon complex triggers a conformational change in the tertiary structure of tRNAPhe from Escherichia coli. A similar conformational transition is demonstrated here in the poly(A)/tRNALys system. C-G-A or C-G-A-A was used as a probe for the conformational transition in tRNA. These probes bound to tRNAPhe and tRNALys more strongly in the presence of the corresponding codons than in the absence. In order to verify these data by an independent method, the decrease in absorbance at 300 nm that occurs on formation of the codon-anticodon complex in tRNALys (which contains 2-thio-5-methylaminomethyluridine, s2mam5U) was used. The binding constants for formation of A3 . tRNALys (Ka = 2.4 . 10(4) M-1) and A4 . tRNALys (Ka = 2.5 . 10(5) M-1) are very close to those obtained by equilibrium dialysis. In the presence of C-G-A the apparent binding constant of A3 to tRNA was raised 10-fold to 2.5 . 10(-5) M-1. It was calculated that the constant for the binding of C-G-A to the binary complex A3 . tRNALys is approximately 2 . 10(4) M-1, whereas binding to the free tRNA is lower than 10(3) M-1. Under appropriate conditions binding of A3 to tRNALys can be induced directly by the addition of C-G-A. These data demonstrate that codon-anticodon complex formation induces a conformational change in the tRNA that as a consequence allows the binding of a trinucleoside diphosphate, presumably to the T-psi-G region.