Thermal-denaturation profiles of helical polynucleotides have been measured in the presence of increasing concentrations of ethidium bromide. The poly(A)·poly(U) helix is strongly stabilized by binding of ethidium, to much the same extent as is DNA, but the stabilizing effect on poly(I)·poly(C) is much smaller. In the poly(A)·2poly(U) system the drug selectively destabilizes and eventually destroys the triple helix, leaving only the double-helix-to-coil transition.
