The interaction of actinomycin with DNA: requirement for the 2-amino group of purines.

The formation of complexes between actinomycin (AM) (Fig. 1) and DNA is known to depend on specific structures in both the antibiotic and the polydeoxynucleotide.' These include the base guanine in the helical conformation of DNA, and the amino group, the quinoidal oxygen, and the intact peptide lactones of the antibiotic. A model for the structure of AM-DNA complexes which accounts for the participation of these functional groups has been proposed.2 According to this model, AM is located in the minor groove of the DNA helix, where three hydrogen bonds may be formed between the chromophore of the antibiotic and deoxyguanosine in DNA. The specificity of the interaction is attributed to a hydrogen bond between the quinoidal oxygen of AM and the 2--amino group of guanine. The model has already been subjected to a variety of tests. For example, several lines of evidence show that the N-7 of guanine is probably not involved in complexing AM, since extensive substitution of this position by mustard gas3 or other alkylating agenlts4 does not diminish the actinomycin binding capacity of DNA; conversely, AM does not affect the rate of alkylation of DNA by mustard gas.' Another experimental test of the model has been conducted with the synthetic DNA polymer dIdC.A The structure of this polymer corresponds exactly to that of dGdC except for the absence of the 2-amino group of guanine. As judged by the results of spectral and enzymatic assays, the removal of this amino group is correlated with a loss of the ability to interact with AM.3 These and other findings are therefore fully in accord with predictions which can be derived from the model. In this paper we describe the results of another experimental test of the model. The principle of the experiments is outlined in Figure 2. Sar As noted above, the removal of the 2-amino group of L-pro L-NL-pro L-NMevoI l Meval guanine from the AM-sensitive G-C base pair yields the D-val L 0 D-val 0 AM-resistant I-C pair. The A-T base pair is known Ith, LIthr not to interact with AM;' however, the structure of co co this base pair permits the insertion of an amino group N>N NH2 at the position in the helix normally occupied by the amino group of guanine. Such an insertion can be