Figure 4.

Stabilization of the DNA double helix in vivo by superhelical stress for temperatures between 273 K and 380 K, where extremophile organisms exist. Although unconstrained DNA (gray dashed line) is unstable at high temperatures, the contribution of the average twist in the free energy (dotted line) maintains the stability of the double helix (black solid line) for an additional 15 K, with an almost constant opening penalty of ∼0.2 kcal/mol. Above a critical temperature T_c ≈ 378 K (dots), the contribution from the twist stiffness of the double helix (dash-dotted line) prevents further stabilization. The superhelical stress opposes the spontaneous decrease of the average twist, and results in an ∼1°/bp increase in the considered interval.