Effects of supercoiling in electrophoretic trapping of circular DNA in polyacrylamide gels

Electrophoretic velocity and orientation have been used to study the electric-field-induced trapping of supercoiled and relaxed circular DNA (2926 and 5386 bp) in polyacrylamide gels (5% T, 3.3% C) at 7.5-22.5 V/cm, using as controls linear molecules of either the same contour length or the same radius of gyration. The circle-specific trapping is reversible. From the duration of the reverse pulse needed to detrap the molecules, the average trap depth is estimated to be 90 A, which is consistent with the molecular charge and the field strengths needed to keep molecules trapped. Trapped circles exhibit a strong field alignment compared to the linear form, and there is a good correlation between the enhanced field alignment for the circles and the onset of trapping in both constant and pulsed fields. The circles do not exhibit the orientation overshoot response to a field pulse seen with linear DNA, and the rate of orientation growth scales as E(-2+/-0.1) with the field, as opposed to E(-1.1+/-0.1) for the linear form. These results show that the linear form migrates by cyclic reptation, whereas the circles most likely are trapped by impalement on gel fibers. This proposal is supported by very similar velocity and orientation behavior of circular DNA in agarose gels, where impalement has been deemed more likely because of stiffer gel fibers. The trapping efficiency is sensitive to DNA topology, as expected for impalement. In polyacrylamide the supercoiled form (superhelical density sigma = -0.05) has a two- to fourfold lower probability of trapping than the corresponding relaxed species, whereas in agarose gels the supercoiled form is not trapped at all. These results are consistent with existing data on the average holes in the plectonemic supercoiled structures and the fiber thicknesses in the two gel types. On the basis of the topology effect, it is argued that impalement during pulsed-field electrophoresis in polyacrylamide gels may be useful for the separation of more intricate DNA structures such as knots. The results also indicate that linear dichroism on field-aligned molecules can be used to measure the supercoiling angle, if relaxed DNA circles are used as controls for the global degree of orientation.