The flexibility of DNA double crossover molecules

Abstract

Double crossover molecules are DNA structures containing two Holliday junctions connected by two double helical arms. There are several types of double crossover molecules, differentiated by the relative orientations of their helix axes, parallel or antiparallel, and by the number of double helical half-turns (even or odd) between the two crossovers. They are found as intermediates in meiosis and they have been used extensively in structural DNA nanotechnology for the construction of one-dimensional and two-dimensional arrays and in a DNA nanomechanical device. Whereas the parallel double helical molecules are usually not well behaved, we have focused on the antiparallel molecules; antiparallel molecules with an even number of half-turns between crossovers (termed DAE molecules) produce a reporter strand when ligated, facilitating their characterization in a ligation cyclization assay. Hence, we have estimated the flexibility of antiparallel DNA double crossover molecules by means of ligation-closure experiments. We are able to show that these molecules are approximately twice as rigid as linear duplex DNA.

FIGURE 1

Schematic drawings of the five different structural arrangements of double crossover structures. The structures shown are named by the acronym describing their basic characteristics. All names begin with “D” for double crossover. The second character refers to the relative orientations of their two double helical domains, “A” for antiparallel and “P” for parallel. The third character refers to the number (modulus 2) of helical half-turns between crossovers, “E” for an even number and “O” for an odd number. A fourth character is needed to describe parallel double crossover molecules with an odd number of helical half-turns between crossovers. The extra half-turn can correspond to a major (wide) groove separation designated by “W”, or an extra minor (narrow) groove separation designated by “N”. The strands are drawn as zigzag helical structures, where two consecutive, perpendicular lines correspond to a full helical turn for a strand. The arrowheads at the ends of the strands designate their 3′ ends. The structures contain implicit symmetry, which is indicated by the conventional markings: a lens-shaped figure (DAE) indicating a potential dyad perpendicular to the plane of the page, and arrows indicating a twofold axis lying in the plane of the page. Note that the dyad in the DAE molecule shown is only approximate if the central strand contains a nick, which destroys the symmetry. The DAE molecule shown contains two turns between crossovers, like the molecules used in this study. An attempt has been made to portray the differences between the major and minor grooves. Note the differences between the central portions of DPOW and DPON. Also note that the symmetry brings symmetrically related portions of backbones into apposition along the center lines in parallel molecules in these projections. The same contacts are seen to be skewed in projection for the antiparallel molecules.

FIGURE 2

Comparison of the closure of the central cyclic 40-mer and 42-mer strands in DAE molecules. This is a denaturing gel autoradiogram that compares ligation efficiencies for two possible species of DAE molecules. Lane 1 contains linear markers (labeled No. L, where No. represents their lengths) and lanes 8 and 9 contain cyclic markers (labeled No. C, where No. again represents their lengths). Lanes 2–4 contain bands derived from the labeled 40-mer, and lanes 5–7 contain bands derived from the labeled 42-mer. Lanes 2 and 5 contain unligated material, lanes 3 and 6 contain the ligation products of these strands in the DAE context, and lanes 4 and 7 contain the results of treating the material in lanes 3 and 6, respectively, with exonuclease III. It is evident that under the conditions used, the 40-mer cyclizes only partially, whereas the 42-mer cyclizes almost completely.

FIGURE 3

The sequences of the molecules used. The three molecules, DAE-41, DAE-42, and DAE-43, are shown. Small arrowheads indicated 3′ ends of strands. The central strand in each was sealed in the experiments, but the preligation 5′ and 3′ ends are indicated. Scission points by restriction enzymes are indicated by filled triangles. Recognition sites for restriction enzymes are indicated by hollow type. Biotin groups are indicated by bold, larger “B” characters. The molecules are prepared by ligating them together to form the structures indicated as topologically closed species. The final molecules to ligate are then prepared by restricting away the hairpin ends. A control linear duplex molecule, LD-42, is also shown in dumbbell form as DB-42. The nicked molecule DAE-42N is the same as the molecule DAE-42, except that its central strand is not sealed.

FIGURE 4

Characterization of the ligatable material. (A) Denaturing gel analysis of DAE preparation. This autoradiogram shows the preparation of DAE-42N, DAE-43, and DAE-42. Lane 1 contains a series of linear markers and lane 10 contains a 42-mer marker. To the right are the expected products reflecting various degrees of enzymatic degradation, with complete degradation indicated by the L42 species at the bottom. The radioactively labeled site is indicated in the undigested molecule. Sites where cleavage has occurred in a particular species are indicated by triangles; note that the linear molecule containing 210 nucleotides can be nicked at any of its restriction sites, not just the one indicated. Bbs I treatment is shown in lanes 2, 5, and 8, and streptavidin bead purification is shown in lanes 3, 6, and 9. It is clear that no impurities remain for any of the species in these lanes. (B) Nondenaturing gel characterization of the same species. Lanes 5, 8, and 11 contain the final materials after streptavidin bead purification. It is evident that they are uncontaminated by larger products.

FIGURE 5

The ligation-closure experiment. The initial purified topologically closed product is shown at the top of the figure. The biotin groups along the lower domain backbone helices are shown outside for clarity. The molecule is restricted, and its biotinylated hairpins are then removed (along with partial digestion products) by treatment with streptavidin beads. The product of this restriction is shown below the unrestricted molecule. The resulting molecules are then ligated to yield a collection of linear and cyclic molecules. A distribution of molecules is obtained, but both the linear and cyclic species are represented here only by hexamers. Note that the cyclic molecule is a catenane, but if the unlabeled strand were nicked, a single-stranded circle would result. Removal of one of the two continuous strands from the linear system would result in a rotaxane. Under denaturing conditions, the strands of the linear molecule are capable of dissociating.

FIGURE 6

A two-dimensional denaturing gel showing the products of ligating DAE-42. The gel has been divided into two portions for clarity. The smaller (primarily linear) molecules are shown in a lower panel, and the rest in an upper panel. Linear products are indicated by the designation Lⁿ, where the number n indicates the number of fundamental (42-mer) repeats are contained in the molecule. Single circles are indicated by the notation C′ⁿ, and cyclic catenated products are indicated as Cⁿ. Note how successive species form arcs on the gel.

FIGURE 7

A two-dimensional denaturing gel showing the products of ligating DAE-42N. As in Fig. 6, the gel is shown in two portions. Linear molecules are indicated by “L” with a superscript indicating their size, single circles indicated by “SC” with a superscript, and double circles are indicated by “DC” with a superscript. The species are readily identified from the gel for purposes of quantitation.

FIGURE 8

Comparison of experiment and calculations for two systems. The top two panels indicate the fit between observed and calculated proportions of linear and cyclic products of various lengths for the DAE-42 system. The filled bars are experimental results, and the open bars are calculated based on the parameters derived from the analysis. The bottom two panels show the same extent of agreement for the LD-42 molecules.

FIGURE 9

Extracted sets of j-factors for the species studied. The j-factors for multimers of different molecules are shown as a function of their length. The second order polynomial fits to the data (solid lines) are used to derive the parameters that characterize the molecules.