Cooperative Binding of Manganese (II) to Transfer RNA

We have measured the binding of manganse ions to tRNA in the concentration range of 1 to 28 bound Mn2+ per tRNA at 23°. The free Mn2+ concentration was monitored by electron spin resonance. The Scatchard plot shows that the first four Mn2+ bind cooperatively, at a free Mn2+ concentration of 3 μM. The cooperativity presumably reflects a conformational change of tRNA. A resulting structure with 6–10 strong binding sites (ks= 1.8 × 105M−1), and 21–25 weak binding sites (kw= 5 × 103 M−1) is suggested.From the bound Mn2+ electron spin resonance we estimate a minimal distance between the strong sites of about 1.5 nm; this shows that the binding sites are spread on the molecule and not clustered together.