Interaction of minor groove binding ligands with long AT tracts.

We have used quantitative DNase I footprinting to examine the ability of distamycin and Hoechst 33258 to discriminate between different arrangements of AT residues, using synthetic DNA fragments containing multiple blocks of (A/T)6or (A/T)10in identical sequence environments. Previous studies have shown that these ligands bind less well to (A/T)4sites containing TpA steps. We find that in (A/T)6tracts distamycin shows little discrimination between the various sites, binding approximately 2-fold stronger to TAATTA than (TA)3, T3A3and GAATTC. In contrast, Hoechst 33258 binds approximately 20-fold more tightly to GAATTC and TAATTA than T3A3and (TA)3. Hydroxyl radical footprinting reveals that both ligands bind in similar locations at the centre of each AT tract. At (A/T)10sites distamycin binds with similar affinity to T5A5, (TA)5and AATT, though bands in the centre of (TA)5are protected at approximately 50-fold lower concentration than those towards the edges. Hoechst 33258 shows a similar pattern of preference, with strong binding to AATT, T5A5and the centre of (TA)5. Hydroxyl radical footprinting reveals that at low concentrations both ligands bind at the centre of (TA)5and A5T5, while at higher concentrations ligand molecules bind to each end of the (A/T)10tracts. At T5A5two ligand molecules bind at either end of the site, even at the lowest ligand concentration, consistent with the suggestion that these compounds avoid the TpA step. Similar DNase I footprinting experiments with a DNA fragment containing T n (n = 3-6) tracts reveals that both ligands bind in the order T3< T4 << T5 = T6.