Mir M A, Dasgupta D
Biophysics Division, Saha Institute of Nuclear Physics, 37 Belgachhia Road, Kolkata-700 037, India.
Biochemistry. 2001 Sep 25;40(38):11578-85. doi: 10.1021/bi010731r.
The anticancer antibiotic chromomycin A(3) is a transcription inhibitor which forms two types of complexes with Mg(2+): complex I (1:1 in terms of chromomycin A(3)-Mg(2+)) and complex II (2:1 in terms of chromomycin A(3)-Mg(2+)). These complexes are the DNA-binding ligands. With the broad objective of elucidation 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-terminal tail domains, in the association of nucleosome with both complexes of chromomycin. From a scrutiny of the spectroscopic features of the two bound complexes and comparison of the binding and associated thermodynamic parameters, we have shown the following. Core histone(s) stand(s) in the way of 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 different. Tryptic removal of N-terminal tail domains of core histones enhances the binding potential and access of both complexes of chromomycin to the nucleosomal DNA. Agarose gel electrophoresis of an equilibrium mixture containing either complex I or complex II and a saturating concentration of the core particle has demonstrated that both complexes have a tendency to disrupt the nucleosome structure, leading to a release of nucleosomal DNA. Compared to the N-terminal intact nucleosome, the N-terminal chopped nucleosome is more susceptible to disruption. Therefore, we suggest from the above results that the N-terminal tail domains, which have an important role in eukaryotic gene expression, stand in the way of a free access of external agents such as anticancer drugs to the eukaryotic genome. The significance of the results to understand the molecular basis of the transcription inhibitory capacity of chromomycin is discussed.
抗癌抗生素嗜铬霉素A(3)是一种转录抑制剂，它与Mg(2+)形成两种类型的复合物：复合物I（嗜铬霉素A(3)-Mg(2+)的比例为1:1）和复合物II（嗜铬霉素A(3)-Mg(2+)的比例为2:1）。这些复合物是DNA结合配体。为了阐明这类转录抑制剂在真核系统中的作用机制这一广泛目标，我们研究了该抗生素在不同条件下与核小体核心颗粒的相互作用。我们已经证明并表征了核心组蛋白，特别是N端尾部结构域，在核小体与嗜铬霉素的两种复合物结合中的作用。通过仔细研究两种结合复合物的光谱特征，并比较结合和相关的热力学参数，我们得到了以下结果。核心组蛋白阻碍了配体与核小体DNA的接触。N端完整和切割的核心颗粒与同一复合物的相互作用不同。两种复合物I和II与同一系统的相互作用模式不同。胰蛋白酶去除核心组蛋白的N端尾部结构域增强了嗜铬霉素两种复合物与核小体DNA的结合潜力和接触。含有复合物I或复合物II以及饱和浓度核心颗粒的平衡混合物的琼脂糖凝胶电泳表明，两种复合物都有破坏核小体结构的趋势，导致核小体DNA的释放。与N端完整的核小体相比，N端切割的核小体更容易被破坏。因此，我们从上述结果中推测，在真核基因表达中起重要作用的N端尾部结构域阻碍了抗癌药物等外部试剂自由进入真核基因组。讨论了这些结果对于理解嗜铬霉素转录抑制能力分子基础的意义。
