Changing the target base specificity of the EcoRV DNA methyltransferase by rational de novo protein-design.

The EcoRV DNA-(adenine-N(6))-methyltransferase (M.EcoRV) specifically modifies the first adenine residue within GATATC sequences. During catalysis, the enzyme flips its target base out of the DNA helix and binds it into a target base binding pocket which is formed in part by Lys16 and Tyr196. A cytosine residue is accepted by wild-type M.EcoRV as a substrate at a 31-fold reduced efficiency with respect to the k(cat)/K(M) values if it is located in a CT mismatch substrate (GCTATC/GATATC). Cytosine residues positioned in a CG base pair (GCTATC/GATAGC) are modified at much more reduced rates, because flipping out the target base is much more difficult in this case. We intended to change the target base specificity of M.EcoRV from adenine-N(6) to cytosine-N(4). To this end we generated, purified and characterized 15 variants of the enzyme, containing single, double and triple amino acid exchanges following different design approaches. One concept was to reduce the size of the target base binding pocket by site-directed mutagenesis. The K16R variant showed an altered specificity, with a 22-fold preference for cytosine as the target base in a mismatch substrate. This corresponds to a 680-fold change in specificity, which was accompanied by only a small loss in catalytic activity with the cytosine substrate. The K16R/Y196W variant no longer methylated adenine residues at all and its activity towards cytosine was reduced only 17-fold. Therefore, we have changed the target base specificity of M.EcoRV from adenine to cytosine by rational protein design. Because there are no natural paragons for the variants described here, a change of the target base specificity of a DNA interacting enzyme was possible by rational de novo design of its active site.