Magnesium-dependent supercoiling-induced transition in (dG) sub(n) multiplied by (dC) sub(n) stretches and formation of a new G-structure by (dG) sub(n) strand

Plasmids containing (dG) sub(27) multiplied by (dC) sub(27) inserts (pPG27), (dG) sub(37) multiplied by (dC) sub(37) inserts (pPG37), and (dG) sub(24) C(dG) sub(21) multiplied by (dC) sub(24) G(dC) sub(21) inserts (pPG46C) were constructed for the study of structural transitions within (dG) sub(n) multiplied by (dC) sub(n) stretches. Two-dimensional gel electrophoresis has shown that a Mg super(2+)-dependent supercoiling-induced structural transition takes place at pH 8 in plasmid pPG46C. The transition occurs at - sigma =0.06 and involves a supercoiling release corresponding to 5 superhelical turns.