Ortiz-Lombardía M, Eritja R, Azorín F, Kypr J, Tejralova I, Vorlícková M
Departament de Biologia Molecular i Cel.lular, Centre d'Investigació i Desenvolupament-CSIC, Barcelona, Spain.
Biochemistry. 1995 Nov 7;34(44):14408-15. doi: 10.1021/bi00044a018.
Homopurine DNA sequences are highly structurally polymorphic. In particular, d(GA)n DNA sequences are known to be capable of forming intramolecular foldbacks, bimolecular homoduplexes, and tetrastranded complexes. Counterions play a determinant role on the equilibria between the different structural conformers of d(GA)n sequences. In this paper, the effect of divalent zinc cations on the structure of a d(GA)20 oligonucleotide has been analyzed by CD spectroscopy and polyacrylamide gel electrophoresis. Depending on the precise experimental conditions at which zinc is added, two distinct conformations of the d(GA)20 oligonucleotide are stabilized. At neutral pH in the absence of zinc, d(GA)20 is partially organized into intramolecular foldbacks and bimolecular homoduplexes [Casasnovas et al. (1993) J. Mol. Biol. 233, 671-681]. Under these conditions, addition of zinc results in the stabilization of the bimolecular homoduplex which is nonspecific for zinc since it is also stabilized by divalent magnesium cations, increasing ionic strength, or decreasing pH. Its CD spectrum is identical to that reported earlier for parallel-stranded d(GA)n homoduplexes [Rippe et al. (1992) EMBO J. 11, 3777-3786]. On the other hand, if zinc is added under conditions where the d(GA)20 oligonucleotide is exclusively single-stranded, a different bimolecular homoduplex appears which is only observed in the presence of zinc. The zinc-specific duplex melts cooperatively, and, in contrast to the nonspecific duplex, its thermostability is high. Transition from the nonspecific to the zinc-specific duplex is observed at high zinc concentrations or at high temperatures. The transition is cooperative. These results are discussed in the context of the specific cation effects on the formation of intramolecular R.R.Y triplexes at d(GA.TC)n DNA sequences.
同嘌呤DNA序列具有高度的结构多态性。特别地，已知d(GA)n DNA序列能够形成分子内回折、双分子同源双链体和四链体复合物。抗衡离子在d(GA)n序列不同结构构象之间的平衡中起决定性作用。在本文中，通过圆二色光谱法和聚丙烯酰胺凝胶电泳分析了二价锌阳离子对d(GA)20寡核苷酸结构的影响。根据添加锌时的精确实验条件，d(GA)20寡核苷酸的两种不同构象得以稳定。在中性pH且无锌的情况下，d(GA)20部分组织成分子内回折和双分子同源双链体[卡萨诺瓦斯等人（1993年）《分子生物学杂志》233卷，671 - 681页]。在这些条件下，添加锌会导致双分子同源双链体的稳定，这种双链体对锌不具有特异性，因为它也能被二价镁阳离子、增加离子强度或降低pH值所稳定。其圆二色光谱与先前报道的平行链d(GA)n同源双链体的光谱相同[里佩等人（1992年）《欧洲分子生物学组织杂志》11卷，3777 - 3786页]。另一方面，如果在d(GA)20寡核苷酸完全为单链的条件下添加锌，则会出现一种不同的双分子同源双链体，这种双链体仅在有锌的情况下才会观察到。锌特异性双链体协同解链，并且与非特异性双链体不同，其热稳定性很高。在高锌浓度或高温下可观察到从非特异性双链体向锌特异性双链体的转变。这种转变是协同的。本文结合特定阳离子对d(GA.TC)n DNA序列分子内R.R.Y三链体形成的影响对这些结果进行了讨论。
