Kang S, Wells R D
Institute of Biosciences and Technology, Texas A & M University, Texas Medical Center, Houston 77030.
J Biol Chem. 1992 Oct 15;267(29):20887-91.
The effect of the central non-Pur.Pyr sequences in oligo(dG.dC) inserts on determining the type of intramolecular DNA triplex isomers formed in negatively supercoiled plasmids was investigated. Different triplex types (H-r3, H-r5, and H-y3), revealed by a combination of chemical probing and Maxam-Gilbert sequencing reactions, were adopted by the oligo(dG.dC) tracts depending on the length and composition of the central non-Pur.Pyr sequences (0, 3, or 5 base pairs) and the kind of metal ions. The H-r3 triplex conformer, one isomer of a Pur.Pur.Pyr structure, was formed in the (C)20 and (C)10GCG(C)10 inserts in plasmids in the presence of certain metal ions. Interestingly, H-r5, the other isomer of the Pur.Pur-Pyr triplex which had not been detected previously, was formed in a (C)9GAATT(C)9 insert in the presence of either Mg2+ or Ca2+. Alternatively, H-y3, one isomer of a Pyr.Pur.Pyr triplex, was formed in the (C)9GAATT(C)9 insert in the absence of metal ions. Thus, central non-Pur.Pyr sequences and metal ions play a role as determinants of the types of intramolecular triplexes formed; they also reduce the requirement of longer Pur.Pyr repeat sequences to form intramolecular triplexes. Furthermore, the effects of MgCl2 concentration and pH on the formation of triplex isomers were examined. The Pur.Pur.Pyr conformations (H-r3 and H-r5) may be the favored conformations in the cellular milieu, since they are stable at physiological pH and metal ion concentration.
研究了寡聚（dG.dC）插入片段中的中央非嘌呤嘧啶序列对确定负超螺旋质粒中形成的分子内DNA三链体异构体类型的影响。通过化学探针和Maxam-Gilbert测序反应相结合揭示的不同三链体类型（H-r3、H-r5和H-y3），被寡聚（dG.dC）片段采用，这取决于中央非嘌呤嘧啶序列的长度和组成（0、3或5个碱基对）以及金属离子的种类。在某些金属离子存在下，质粒中的（C）20和（C）10GCG（C）10插入片段中形成了Pur.Pur.Pyr结构的一种异构体H-r3三链体构象。有趣的是，在Mg2+或Ca2+存在下，（C）9GAATT（C）9插入片段中形成了Pur.Pur-Pyr三链体的另一种异构体H-r5，而此前未检测到该异构体。另外，在没有金属离子的情况下，（C）9GAATT（C）9插入片段中形成了Pyr.Pur.Pyr三链体的一种异构体H-y3。因此，中央非嘌呤嘧啶序列和金属离子作为分子内三链体形成类型的决定因素发挥作用；它们还降低了形成分子内三链体对更长嘌呤嘧啶重复序列的需求。此外，还研究了MgCl2浓度和pH对三链体异构体形成的影响。Pur.Pur.Pyr构象（H-r3和H-r5）可能是细胞环境中有利的构象，因为它们在生理pH和金属离子浓度下是稳定的。
