The intramolecular poly(dG)-poly(dG)-poly(dC) triple helix structure forms at a neutral pH in the presence of Mg++ ions
1990
Kohwi-Shigematsu, T. | Kohwi, Y.
Poly(dG)-poly(dC) sequences, when studied in supercoiled plasmid DNA revealed a capability of inducing altered DNA conformation upon neighboring DNA region in a sequence independent and unidirectional manner. With newly devised chemical cleavage method to map DNA bases modified with chloroacetaldehyde, a chemical that reacts with unpaired DNA bases, we found that poly(dG)-poly(dC) sequences in the presence of Mg++ ions, adopt a tetra-stranded structure containing a dG.dG.dC triple helix structure. The dG.dG.dC triplex is a Mg++ induced structure and forms at a neutral pH as well as acidic pH. In the absence of Mg++, a protonation-induced dC.dG.dC triplex is formed under acidic conditions by the same poly(dG)-poly(dC) sequences. A 16 contiguous dG residues in the 5' flanking sequence of the chicken adult beta-alpha globin gene also adopts a dG.dG.dC triplex structure at neutral pH in the presence of Mg++ ions. Other cationic metals, such as Ca++ and Mn++ ions also induce a dG.dG.dC triplex structure at the 16 contiguous residues whereas Zn++, Co++, and Cu++ ions do not promote the triplex structure formation.
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