Mir Mohd Ayoub, Das Suman, Dasgupta Dipak
Biophysics Division, Saha Institute of Nuclear Physics, 37 Belgachhia Road, Calcutta 700 037, India.
Biophys Chem. 2004 Apr 1;109(1):121-35. doi: 10.1016/j.bpc.2003.10.023.
Mithramycin (MTR) and daunomycin are two anticancer drugs that bind reversibly to double stranded DNA with (G.C) base specificity leading to inhibition of transcription. MTR is a groove binder of DNA in the presence of a divalent cation such as Mg(2+), while daunomycin intercalates in the double stranded DNA structure. In order to understand the mechanism of action of the two types of transcription inhibitor, namely, groove binder and intercalator, we have studied the effect of N-terminal tail domains in histone proteins of the nucleosome upon the association of both MTR and daunomycin with the nucleosome core particle, because the tails modulate the accessibility to nucleosome during gene expression. Using a combination of spectroscopic, thermodynamic and biochemical studies, we have shown that N-terminal intact and chopped core particles interact differently with the same ligand and the N-terminal tail domains of core histones in the nucleosome stand in the way of free access of these ligands to the nucleosomal DNA. Tryptic removal of N-terminal tail domains of core histones enhances the binding potential and accessibility of both MTR and daunomycin to nucleosomal DNA. They disassemble the nucleosome structure leading to a release of DNA, N-terminal chopped nucleosomes being more susceptible for disruption compared to N-terminal intact nucleosomes. The extent of these effects is more pronounced in case of the intercalator daunomycin. Thus, N-terminal tail domains protect the eukaryotic genome from external agents, such as anticancer drugs, and the degree of protection is dependent upon the mode of binding to DNA.
光神霉素（MTR）和柔红霉素是两种抗癌药物，它们以（G.C）碱基特异性与双链DNA可逆结合，从而抑制转录。在二价阳离子如Mg(2+)存在的情况下，MTR是DNA的沟结合剂，而柔红霉素则插入双链DNA结构中。为了了解这两种转录抑制剂（即沟结合剂和嵌入剂）的作用机制，我们研究了核小体组蛋白中N端尾部结构域对MTR和柔红霉素与核小体核心颗粒结合的影响，因为这些尾部在基因表达过程中调节对核小体的可及性。通过结合光谱学、热力学和生物化学研究，我们表明N端完整和切割的核心颗粒与相同配体的相互作用不同，并且核小体中核心组蛋白的N端尾部结构域阻碍了这些配体自由进入核小体DNA。胰蛋白酶去除核心组蛋白的N端尾部结构域增强了MTR和柔红霉素与核小体DNA的结合潜力和可及性。它们会拆解核小体结构导致DNA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。这些效应在嵌入剂柔红霉素的情况下更为明显。因此，N端尾部结构域保护真核基因组免受外部因子（如抗癌药物）的影响，并且保护程度取决于与DNA的结合方式。
