DNA-sequence binding preference of the GC-selective ligand mithramycin. Deoxyribonuclease-I/deoxyribonuclease-II and hydroxy-radical footprinting at CCCG, CCGC, CGGC, GCCC and GGGG flanked by (AT)n and An.Tn.

We have used hydroxy-radical and deoxyribonuclease-I footprinting to probe the interaction of mithramycin with DNA fragments containing the sequences (AT)10X(AT)10 (X = CCCG, CCGC or CGGC) and A14GCCCT15. As expected the drug produces clear footprints located around the central four GC base pairs. The exact position of the footprint is different for the four sequences; the footprint with CCCG is displayed by two base pairs in the 5' direction relative to GCCC. These variations are explained by suggesting that mithramycin avoids the dinucleotide CG and binds better to GG/CC than GC. Although there is little change in deoxyribonuclease-I cleavage of the surrounding blocks of (AT)n, cleavage by deoxyribonuclease II is markedly enhanced and certain thymines on the 5' side of the ligand-binding site become hyperreactive to hydroxy-radical attack. Adjacent regions of An.Tn show enhanced rates of deoxyribonuclease-I cleavage in the presence of the antibiotic.