Binding of bifunctional ethidium intercalators to transfer RNA

Two bifunctional intercalating dimers, an ethidium homodimer and an acridine ethidium heterodimer, bind to yeast tRNA^phe through two classes of sites, I and II (K_I ≥ 10⁹ M^?1, K_II ～ 10⁶ M^?1), as indicated by fluorescence titration, fluorescence lifetime, “contact” energy transfer and equilibrium dialysis measurements. Binding appears to involve mono-intercalation of the phenanthridinium moiety of these dimers and it is sensitive to, or possibly coupled with, conformational changes within the tRNA macromolecule. These observations raise the possibility that tRNA may represent a pharmacological target of the bifunctional intercalators.