Association of the anticancer antibiotic chromomycin A(3) with the nucleosome: Role of core histone tail domains in the binding process

The anticancer antibiotic chromomycin A(3) is a transcription inhibitor whi ch forms two of complexes with Mg2+: complex I (1:1 in terms of chromomycin A(3)-Mg2+) and complex II (2:1 in terms of chromomycin A(3)-Mg2+). These c omplexes are the DNA-binding ligands. With the broad objective of elucidati on of the mechanism for action of this group of transcription inhibitors in eukaryotic systems, we have studied the interaction of the antibiotic with nucleosome core particles under different conditions. We have demonstrated and characterized the role of core histone proteins, particularly the N-te rminal tail domains, in the association of nucleosome with both complexes o f chromomycin. From a scrutiny of the spectroscopic features of the two bou nd complexes and comparison of the binding and associated thermodynamic par ameters, we have shown the following. Core histone(s) stand(s) in the way o f access of the ligand(s) to nucleosomal DNA. N-Terminal intact and chopped core particles interact differentially with the same complex. The modes of interaction of the two complexes, I and II, with the same system are diffe rent. Tryptic removal of N-terminal tail domains of core histories enhances the binding potential and access of both complexes of chromomycin to the n ucleosomal DNA. Agarose gel electrophoresis of an equilibrium mixture conta ining either complex I or complex II and a saturating concentration of the core particle has demonstrated that both complexes have a tendency to disru pt the nucleosome structure, leading to a release of nucleosomal DNA. Compa red to the N-terminal intact nucleosome, the N-terminal chopped nucleosome is more susceptible to disruption. Therefore, we suggest from the above res ults that the N-terminal tail domains, which have an important role in euka ryotic gene expression, stand in the way of a free access of external agent s such as anticancer drugs to the eukaryotic genome. The significance of th e results to understand the molecular basis of the transcription inhibitory capacity of chromomycin is discussed.