Extrusion of an imperfect palindrome to a cruciform in superhelical DNA: complete determination of energetics using a statistical mechanical model

We present a detailed study of the extrusion of an imperfect palindrome, derived from the terminal regions of vaccinia virus DNA and contained in a superhelical plasmid, into a cruciform containing bulged bases. We monitor the course of extrusion by two-dimensional gel electrophoresis experiments as a function of temperature and linking number. We find that extrusion pauses at partially extruded states as negative superhelicity increases. To understand the course of extrusion with changes in linking number, ΔLk, we present a rigorous semiempirical statistical mechanical analysis that includes complete coupling between ΔLk, cruciform extrusion, formation of extrahelical bases, and temperature-dependent denaturation. The imperfections in the palindrome are sequentially incorporated into the cruciform arms as hairpin loops, single unpaired bases, and complex local regions containing several unpaired bases. We analyze the results to determine the free energies, enthalpies and entropies of formation of all local structures involved in extrusion. We find that, for each unpaired structure, the ΔG, ΔH and ΔS of formation are all approximately proportional to the number of unpaired bases contained therein. This surprising result holds regardless of the arrangement or composition of unpaired bases within a particular structure. Imperfections have major effects on the overall energetics of cruciform extrusion and on the course of this transition. In particular, the extent of the ΔLk change necessary to extrude an imperfect palindrome is considerably greater than that required for extrusion of the underlying perfect palindrome. Our analysis also suggests that, at higher temperatures, significant denaturation at the base of an imperfect cruciform can successfully compete with extension of the cruciform arms.