Mg2+-induced triplex formation of an equimolar mixture of poly(rA) and poly(rU)

AUTOR(ES)

Kankia, Besik I.

FONTE

Oxford University Press

RESUMO

Magnesium ions strongly influence the structure and biochemical activity of RNA. The interaction of Mg2+ with an equimolar mixture of poly(rA) and poly(rU) has been investigated by UV spectroscopy, isothermal titration calorimetry, ultrasound velocimetry and densimetry. Measurements in dilute aqueous solutions at 20°C revealed two differ ent processes: (i) Mg2+ binding to unfolded poly(rA)·poly(rU) up to [Mg2+]/[phosphate] = 0.25; and (ii) poly(rA)·2poly(rU) triplex formation at [Mg2+]/[phosphate] between 0.25 and 0.5. The enthalpies of these two different processes are favorable and similar to each other, ∼–1.6 kcal mol–1 of base pairs. Volume and compressibility effects of the first process are positive, 8 cm3 mol–1 and 24 × 10–4 cm3 mol–1 bar–1, respectively, and correspond to the release of water molecules from the hydration shells of Mg2+ and the polynucleotides. The triplex formation is also accompanied by a positive change in compressibility, 14 × 10–4 cm3 mol–1 bar–1, but only a small change in volume, 1 cm3 mol–1. A phase diagram has been constructed from the melting experiments of poly(rA)·poly(rU) at a constant K+ concentration, 140 mM, and various amounts of Mg2+. Three discrete regions were observed, corresponding to single-, double- and triple-stranded complexes. The phase boundary corresponding to the transition between double and triple helical conformations lies near physiological salt concentrations and temperature.

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