A PHASE-DIAGRAM FOR SODIUM AND POTASSIUM-ION CONTROL OF POLYMORPHISM IN TELOMERIC DNA

Switching between antiparallel and parallel quadruplex structures of t elomeric DNA under the control of intracellular Na+ and K+ has been im plicated in the pairing of chromosomes during meiosis. Using Raman spe ctroscopy we have determined the dependence of the interquadruplex equ ilibrium of the telomeric repeat of Oxytricha nova, upon solution conc entrations of Na+ and K+. Both alkali cations facilitate the formation of an antiparallel foldback quadruplex at low concentration, and a pa rallel extended quadruplex at higher concentration. However, K+ is mor e effective than Na+ in inducing the parallel association. We propose a phase diagram relating d(T(4)G(4))(4) Polymorphism to intracellular [Na+]/[K+] ratios. The phase diagram indicates that the interquadruple x equilibrium is highly sensitive to changes in the mole fraction of e ither cation when the total concentration falls within the interval 65 to 225 mM, a range which encompasses total of the Na+ and K+ concentr ations occurring in a typical mammalian cell. These results support a role for the guanine-rich overhang of eukaryotic DNA in promoting chro mosome association during meiotic synapsis.