Protonated pyrimidine-purine-purine triplex

Abstract

We have studied a protonated pyrimidine-purine-purine (Py-Pu-Pu) triplex, which is formed between the d(C)nd(G)n duplex and the d(AG)m ollgonucleotide as the third strand and carries the CG*A+ protonated base-triads. We have observed such an Intermolecular complex between a plasmid carrying the d(C)18 d(G)18 insert and the d(AG)5 ollgonucleotide without bivalent cations In 200 mM of Na+ at pH4.0. Bivalent cations additionally stabilize the complex. We propose the structures for nearly Isomorphous base-triads TA*A, CG*G and CG*A+ . To identify the H-DNA-IIke structure, which includes the triplex between d(C)n d(G)n duplex and the AG-strand, we have cloned in a superhellcal plasmid the insert: G10TTAA(AG)5. The data on photofootprintlng and chemical modification with diethyl pyrocarbonate, potassium permanganate and dimethyl sulfate demonstrate that the H-like structure with triplex carrying CG*G and CG*A+ base triads is actually formed under add conditions. In the course of this study we have come across unexpected results on probing of Py-Pu-Pu triplexes by dimethyl sulfate (DMS): the protection effect Is observed not only for guanines entering the duplex but also for guanlnes in the third strand lying in the major groove. We have demonstrated this effect not only for the case the novel protonated Py-Pu-Pu triplex but also for the traditional non-protonated Py-Pu-Pu intramolecular triplex (H*-DNA) formed by the d(C)37 d(G)37 Insert in supercolled plasmld In the presence of Mg2+ ions. © 1993 Oxford University Press.