STRETCHED AND OVERWOUND DNA FORMS A PAULING-LIKE STRUCTURE WITH EXPOSED BASES

We investigate structural transitions within a single stretched and su percoiled DNA molecule. With negative supercoiling, for a stretching f orce >0.3 pN, we observe the coexistence of B-DNA and denatured DNA fr om sigma approximate to -0.015 down to sigma = -1. Surprisingly, for p ositively supercoiled DNA (sigma > +0.037) stretched by 3 pN, we obser ve a similar coexistence of B-DNA and a new highly twisted structure. Experimental data and molecular modeling suggest that this structure h as approximate to 2.62 bases per turn and an extension 75% larger than B-DNA. This structure has tightly interwound phosphate backbones and exposed bases in common with Pauling's early DNA structure [Pauling, L . & Corey, R. B. (1953), Proc. Natl. Acad. Sci. USA 39, 84-97] and an unusual structure proposed for the Pf1 bacteriophage