Biochemical and structural characterization of a DNA N6-adenine methyltransferase from Helicobacter pylori

DNA N-6-methyladenine modification plays an important role in regulating a variety of biological functions in bacteria. However, the mechanism of sequence-specific recognition in N-6-methyladenine modification remains elusive. M1.HpyAVI, a DNA N-6-adenine methyltransferase from Helicobacter pylori, shows more promiscuous substrate specificity than other enzymes. Here, we present the crystal structures of cofactor-free and AdoMet-bound structures of this enzyme, which were determined at resolutions of 3.0 angstrom and 3.1 angstrom, respectively. The core structure of M1.HpyAVI resembles the canonical AdoMet-dependent MTase fold, while the putative DNA binding regions considerably differ from those of the other MTases, which may account for the substrate promiscuity of this enzyme. Site-directed mutagenesis experiments identified residues D29 and E216 as crucial amino acids for cofactor binding and the methyl transfer activity of the enzyme, while P41, located in a highly flexible loop, playing a determinant role for substrate specificity. Taken together, our data revealed the structural basis underlying DNA N-6-adenine methyltransferase substrate promiscuity.