双股寡聚d(G-C)6 DNA Z-构象的形成及其构象转换的动态研究

摘要

Synthetic oligo-d(G-C)6 was induced to undergo B-Z conformation transition by appropriate high-salt solution as shown by ultraviolet and circular dichroism spectroscopy. Among the salts tested, MgCl2 had the most potent ability to induce the conformation transition of oligo-d(G-C)6, NaClO4 (5 mol/L) could also induce the full transition and NaCl (5 mol/L) induced only an incomplete transition (Ca. 70%). The effect of temperature on the B-Z conformation transition of oligo-d(G-C)6 varied with the kind of salts used. On the basis of the observation, duodecandeoxyribonucleotide-d(GpCpGpCpGpCpGpCpGpCpGpC) was the minimal sequence that would support left-handed conformation in aqueous salt solution. And the results show that, oligo-d(G-C)6, or oligo-d(C-G)6 in 4 mol/L MgCl2 solution, the process of its conformation transition might be:B(=)Z(=)X,"X",an unknown conformation form is present,it may be very helpful to make a complete search for the Biological function of Z-DNA.%研究揭示,生物信息的形成,传递与DNA构象的多样性,特别是其中的左手螺旋Z-构象DNA(Z-DNA)相关.在机体DNA链中,普遍存在的特异序列结构d(C-G)n和d(G-C)n片段易形成Z-构象.但对d(G-C)n序列结构的寡聚体Oligo-d(G-C)n,(n小于8)能转换形成Z-DNA片段少见报道.为促进对Z-DNA尤其是其中的短片段Z-DNA与生物功能的相关性研究,我们对合成并纯化后的寡聚体Oligo-d(G-C)n,n分别为4,6,8,10, 及Oligo-d(C-G)6和多聚体poly-d(G-C)500-900进行Z-构象的形成和其构象转换的比较研究.研究结果发现:①d(GpCpGpCpGpCpGpCpGpCpGpC)是d(G-C)n序列结构中能转换形成Z-构象的最短片段(n＝6).其转换成Z-构象能力有链长依赖性(poly d(G-C)500-900易于Oligo-d(G-C)6);②Oligo-d(G-C)6的Z-构象形成能力因溶液中的介质性质不同而异.Co(NH3)3+>Mg2+>Na+;C1O-4>Cl-,因此要求盐溶液的浓度差异很大.③PH7.2,室温条件下,在MgCl2, NaClO4, NaCl溶液浓度分别由0 mol/L增至6.0 mol/L,Oligo-d(G-C)6的B、Z构象转换都出现:B-构象相对稳定期,B-、Z-构象转换跃迁期和Z-构象相对稳定期.每个阶段要求跨越的盐浓度变迁范围也因所用介质而异.当溶液中Oligo-d(G-C)6 B-构象、Z-构象各占50%(θ1/2)时,其盐浓度分别为1.72 mol/L(MgCl2),2.88 mol/L(NaClO4),3.85 mol/L(NaCl).④Oligo-d(G-C)6的B-,Z-构象转换程度受盐浓度影响:当Oligo-d(G-C)6处于最适条件和不同盐溶液其浓度为θ(12)浓度时,温度由8 ℃→22 ℃,在MgCl2,NaClO4溶液中的Oligo-d(G-C)6形成Z-构象能力增加,当由22 ℃→60 ℃,MgCl2溶液中的Z-构象Oligo-d(G-C)6加速增加,而在NaClO4溶液中则是急速向B-型Oligo-d(G-C)6方向转换;温度变化对处于NaCl溶液中的Oligo-d(G-C)6B-、Z-构象相对平衡影响较小.⑤甲基化胞嘧啶即Oligo-d(G-mC)6或d(mC-G)6均增大Z-构象形成能力.⑥在4 mol/L MgCl2溶液中的Oligo-d(G-C)6或Oligo-d(C-G)6或poly d(G-C)500-900的UVab谱、UVcd谱均显示出非B-型或Z-型DNA的新谱型.并且有链长依赖性和因溶液浓度改变出现构象可逆性转变.提示在Oligo-d(G-C)6的构象转换过程中可能存在新构象"X"型,即BZX构象转换模式.