The ends of eukaryotic chromosomes contain specialized structures that include DNA with multiple tandem repeats of simple sequences containing clusters of G on one strand, together with proteins which synthesize and bind to these sequences. The unit repeat in the protozoan Oxytricha with the cluster dT4G4can form structures containing tetrads of guanine residues, referred to G4DNA, in the presence of metal ions such as Na+or K+. We show here that, in the presence of Na+, dT4G4forms a tetramer with parallel strands by means of a UV cross-linking assay. In the presence of K+, two further interactions are observed: at low temperature, higher order complexes are formed, provided the 3ʹ end of the strand is G; a single 3ʹT inhibits this association in dT4G4T. At high temperature, these complexes dissociate, leading to a tetramer with a different ordered structure that melts only at very high temperatures. These results suggest that the cohesive properties of DNA containing G clusters might depend on associative interactions driven by a free 3ʹG terminus in the presence of K+, as well as by connecting antiparallel G hairpins as has been postulated.
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