Supercoiling-induced structural change in the AGGAG repeat detected by chemical modification. Reactions of the Maxam-Gilbert type were carried out on relaxed and supercoiled pSK-(AGGAG)22 and analyzed by sequencing gel. (a) Oxidation of T residues by permanganate. Lane 1: Maxam-Gilbert C reaction; lane 2: relaxed DNA; lane 3: supercoiled DNA. The large bracket indicates the repeat and the small one, the 5′-most CTCCT unit. (b) Modification of A residues by diethylpyrocarbonate. Lane 1: Maxam-Gilbert G reaction; lane 2: supercoiled DNA; lane 3: relaxed DNA. The large bracket indicates the repeat and the small one, the 5′-most AGGAG unit. (c) An H-r3 structure consistent with the modification results. Filled circles indicate Watson-Crick base pairing, and open circles indicate triplex formation. Triangles indicate the bases expected to be reactive to the chemicals.
