Electrostatic interactions and the folding of the four-way DNA junction: analysis by selective methyl phosphonate substitution.

The structure and dynamics of the four-way (Holliday) junction are strongly dependent on the presence of metal ions. In this study, the importance of phosphate charge in and around the point of strand exchange has been explored by selective replacement with electrically neutral methyl phosphonate groups, guided by crystal structures of the junction in the folded, stacked X conformation. Junction conformation has been analysed by comparative gel electrophoresis and fluorescence resonance energy transfer (FRET). Three of sets of phosphate groups on the exchanging strands have been analysed; those at the point of strand exchange and those to their 3' and 5' sides. The exchanging and 3' phosphate groups form a box of negatively charged groups on the minor groove face of the junction, while the 5' phosphate groups face each other on the major groove side, with their proR oxygen atoms directed at one another. The largest effects are observed on substitution of the exchanging phosphate groups; replacement of both groups leads to the loss of the requirement for addition of metal ions to allow junction folding. When the equivalent phosphate groups on the continuous strands were substituted, a proportion of the junction folded into the alternative conformer so as to bring these phosphate groups onto the exchanging strands. These species did not interconvert, and thus this is likely to result from the alternative diasteromeric forms of the methyl phosphonate group. This shows that some of the conformational effects result from more than purely electrostatic interactions. Smaller but significant effects were observed on substitution of the flanking phosphate groups. All methyl phosphonate substitutions at these positions allowed folding to proceed at a reduced concentration of magnesium ions, with double substitutions more effective than single substitutions. Substitution of 5' phosphates resulted in a greater degree of folding at a given ionic concentration compared to the corresponding 3' phosphate substitutions. These results show that the phosphate groups at the point of strand exchange exert the largest electrostatic effect on junction folding, but a number of phosphate groups in the vicinity of the exchange region contribute to the overall effects.