Crystal Structure of the HhaI DNA Methyltransferase

DNA methyltransferases (MTases) exist in diverse organisms ranging from bacteria to mammals (Adams 1990). As a component of restriction and modification systems in bacteria, MTases methylate the DNA of the host cell and protect it from degradation by the cognate endonuclease, whose main function is believed to be the destruction of foreign DNA (Roberts and Halford 1993). Among type II restriction and modification systems, MTases recognize specifically four to eight base pairs of DNA, usually in palindromic sequences where methylation occurs symmetrically (Roberts and Halford 1993). MTases require S-adenosyl-L-methionine (AdoMet) as their methyl donor and act mainly as monomers. Because a fairly large number of MTases with diverse specificities are available, they offer exceptional opportunities for understanding how enzymes recognize and modify DNA.