Binding of Actinomycin D to the T(G)nT Motif of Double-Stranded DNA: Determination of the Guanine Requirement in Nonclassical, Non-GpC Binding Sites

Abstract

Strong binding of the antitumor antibiotic actinomycin D to the sequence 5'-TGGGT-3' in double-stranded DNA was recently established by equilibrium binding studies (Bailey et al., 1993). Actinomycin D binding to this -TGGGT- containing sequence was shown to be comparable to that of an -XGCY- containing oligonucleotide (Ka approximately 10(6) M-1). Investigation of -TGGGT- as a high-affinity binding site for actinomycin D follows from our 1989 sequencing study (Rill et al., 1989) in which the photoaffinity analog of actinomycin D (7-azidoactinomycin D) was used to determine DNA base sequence specificities and neighboring base effects. The studies presented here examine the guanine requirements for actinomycin D binding to such nonclassical (non-dGpC) sites by varying the number of central guanine residues in a series of selected duplex oligonucleotides. The central -T(G)nT- motif varies from n equals 1 to 4. Actinomycin D binding to each of these undecamers is characterized and correlated with binding to oligonucleotides of identical length and similar sequences that contain classical dGpC binding sites. Binding affinities of actinomycin D to this series of oligonucleotide duplexes (10 degrees C) can be summarized as -TGGGT- > -TGGT- > TGGGGT- > -TGT. The kinetics of SDS-induced dissociation of actinomycin D from these oligonucleotides reveal single-exponential decays with duration dependent on the sequence at the binding site. With the exception of the -TGGGT- containing oligomer, dissociation times for the T(G)nT duplexes were drastically different and much shorter than times obtained for the dissociation of actinomycin D from oligonucleotides having classical dGpC sites.(ABSTRACT TRUNCATED AT 250 WORDS)