Binding of actinomycin C 3 to mono, di, and oligonucleotides

Abstract

Sequence specificity in the binding of the actinomycins to DNA has been studied by measuring the dissociation constants of several deoxyribodinucleotide‐actinomycin complexes by spectral‐titration. Additional data for some mononucleotides and for a deoxyribotetranucleotide have been collected for comparison. The results show that:G‐Containing dinucleotides form complexes of roughly 10 times higher stability than G‐free dinucleotides. These data parallel the corresponding results for mononucleotides [16].The G‐containing dinucleotides may be subdivided into three classes of the following sequential types:a) d(pX‐G), which bind the actinomycins with similar affinity as the dGMP,b) d(pG‐X), which bind the actinomycins less strongly than dGMP, andc) d(pG‐C), which binds the actinomycins in a 2:1 complex (two dinucleotides per dye molecule) of higher stability than all the other dinucleotides.The tetranucleotide d(pC‐T‐A‐G) behaves as the dinucleotides of type a.Molecular weights of the complexes as determined by sedimentation equilibrium measurements confirmed the stoichiometry of the actinomycin‐d(pG‐C) complex obtained by titration. Corresponding measurements for the actinomycin complexes of d(pC‐G) and d(pC‐T‐A‐G) revealed that the antibiotic strongly favours double helix formation of these self complementary oligonucleotides, yielding 2:2 complexes with two equivalent binding sites at the ends of the helices.All data are consistent with fixation of the actinomycin chromophore on the 3′‐side of the dG‐residue as in the actinomycin‐dG crystal [8].