Spontaneous base flipping in DNA and its possible role in methyltransferase binding.

Recent crystallographic studies showed that HhaI and other methyltransferases flip their target DNA base completely out of a DNA helix. This base flipping is also a key feature in a number of other enzyme-catalyzed processes involving DNA. The mechanism of base flipping by these enzymes remains elusive. Based on a full atomic level description of bond rotational motions we have studied the energetics of flipping a base in a B-DNA duplex in the absence of the enzyme. We have also investigated the effect of the restraints from enzyme-distorted DNA backbone on the movement of a flipped base in several methytransferase bound DNA crystal structures. Our study on crystal B-DNA helices showed that a base could be flipped at an energy cost close to the enthalpy observed for base pair opening in premelting thermal fluctuations. This suggests that spontaneous base flipping in DNA due to thermal fluctuation may be achieved. Analysis of several crystal HhaI and HaeIII methyltransferase DNA duplex structures showed that the enzyme induced DNA backbone distortion severely restricts the movement of the flipped base, which indicates that during base flipping the backbone needs to adopt a substantially different conformation than that observed in the x-ray (enzyme-bound) structures. Our results suggest the possible role of thermally induced transient base opening in facilitating recognition and binding of methyltransferases and other enzymes.