Facile transition of poly[d(TG) x d(CA)] into a left-handed helix in physiological conditions

The DNA polymer d(GC)n . d(GC)n can undergo a transition from the usual right-handed 10.4 base pairs (bp) per turn B form to a novel left-handed 12 bp per turn Z form in response to altered environmental conditions. Several other alternating purine-pyrimidine DNA polymers with modified bases have been shown to undergo transitions from B to Z conformations, with varying degrees of difficulty. We report here that the unmodified DNA polymer d(TG)n . d(CA)n readily undergoes a transition to a Z conformation when subjected to unwinding torsional stress in ionic conditions that are close to physiological. By using a two-dimensional gel electrophoresis system, we have determined both the critical free energy of supercoiling that is required to initiate the transition and the free energy of supercoiling that is required to maintain this polymer in the Z form.