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87061 
Journal Article 
Divalent transition metal cations counteract potassium-induced quadruplex assembly of oligo(dG) sequences 
Blume, SW; Guarcello, V; Zacharias, W; Miller, DM 
1997 
1 
Nucleic Acids Research
ISSN: 0305-1048
EISSN: 1362-4962 
25 
3 
617-625 
English 
9016604 
WOS:A1997WG77100024 
Nucleic acids containing tracts of contiguous guanines tend to self- associate into four-stranded (quadruplex) structures, based on reciprocal non-Watson-Crick (G*G*G*G) hydrogen bonds. The quadruplex structure is induced/stabilized by monovalent cations, particularly potassium. Using circular dichroism, we have determined that the induction/stabilization of quadruplex structure by K+is specifically counteracted by low concentrations of Mn2+(4-10 mM), Co2+(0.3-2 mM) or Ni2+(0.3-0.8 mM). G-Tract-containing single strands are also capable of sequence-specific non-Watson-Crick interaction with d(G. C)-tract- containing (target) sequences within double-stranded DNA. The assembly of these G*G.C-based triple helical structures is supported by magnesium, but is potently inhibited by potassium due to sequestration of the G-tract single strand into quadruplex structure. We have used DNase I protection assays to demonstrate that competition between quadruplex self-association and triplex assembly is altered in the presence of Mn2+, Co2+or Ni2+. By specifically counteracting the induction/stabilization of quadruplex structure by potassium, these divalent transition metal cations allow triplex formation in the presence of K+and shift the position of equilibrium so that a very high proportion of triplex target sites are bound. Thus, variation of the cation environment can differentially promote the assembly of multistranded nucleic acid structural alternatives.