Association of anticancer drug mithramycin with H1-depleted chromatin: a comparison with native chromatin

Depletion of histone H1 after covalent modification from chromatin is a key step in eukaryotic transcription initiation. We have studied the effect of depletion of linker histone H1 upon the association of transcription inhibitor, (mithramycin) 2:Mg 2+ complex, with chromatin. We have compared the binding characteristics of the above complex with native, H1-depleted chromatin and naked DNA. Binding site size (number of bases per ligand molecule) of the above complex to the chromosomal DNA increases upon removal of histone H1. It implies an increase in the accessibility of the ligand for the linker DNA. Spectroscopic data, and associated enthalpy and entropy values of the interaction of the complex with H1-depleted chromatin are similar to naked DNA rather than native chromatin. These results suggest that under in vivo conditions, depletion of histone H1 from transcriptionally inert native chromatin during gene activation would lead to an enhanced accessibility of linker DNA to the small ligands with the potential to inhibit transcription.