Klysik J
Institute of Biosciences and Technology, Texas A&M University, Houston 77030-3303.
J Mol Biol. 1995 Feb 3;245(5):499-507. doi: 10.1006/jmbi.1994.0041.
A plasmid insert containing the (TA)7GATC(TA)7 inverted repeat and an adjacent (AG)7 tract adopts a cruciform structure at neutral pH. However, under acidic conditions the cruciform becomes readily disrupted in favor of a triplex. The (AG)7.(CT)7 duplex and one strand of the (TA)7GATC(TA)7 element are engaged in the three-stranded DNA formation, as determined using single-stranded DNA specific probes. The structure extrudes by displacing the (TA)7 strand adjacent to the (AG)7, and folding it back into the major groove of the (AG)7.(CT)7 duplex. This new variant of H-DNA is supercoil and pH dependent, requiring pH 6.1 or lower to form. The triplex is stabilized by T:A.T and A+:G.C triads. This unusual triad composition (50% of T:A.T and 50% of A+:G>C), that has not previously been observed in intramolecular triplexes, violates both the widely accepted division of triplexes into Py:Pu.Py or Pu:Pu.Py types and the requirement for mirror repeat symmetry.
一个含有(TA)7GATC(TA)7反向重复序列和相邻(AG)7序列的质粒插入片段在中性pH条件下会形成十字形结构。然而，在酸性条件下，十字形结构很容易被破坏，转而形成三链体。如使用单链DNA特异性探针所确定的那样，(AG)7.(CT)7双链体和(TA)7GATC(TA)7元件的一条链参与了三链DNA的形成。该结构通过取代与(AG)7相邻的(TA)7链并将其折叠回(AG)7.(CT)7双链体的大沟中而挤出。这种新的H-DNA变体是超螺旋且依赖于pH值的，需要pH 6.1或更低才能形成。三链体通过T:A.T和A+:G.C三联体得以稳定。这种不寻常的三联体组成（50%为T:A.T，50%为A+:G.C），此前在分子内三链体中未曾观察到，它既违反了被广泛接受的将三链体分为Py:Pu.Py或Pu:Pu.Py类型的分类方式，也违反了对镜像重复对称性的要求。
