Abstract: The present invention provides a novel approach to the production of restriction endonucleases. More specifically, there is provided a novel method for the overexpression of these enzymes, which comprises transforming a host cell with an expression vector containing DNA coding for the restriction endonuclease of interest and a vector for the expression of a methylase which is capable of protecting the host cell DNA against the restriction endonuclease of interest. The methylase vector is compatible with the restriction endonuclease expression vector. Preferably, the expression vector is a T7 expression vector and the cell is transformed with a third compatible vector which regulates the expression vector. Also disclosed is the cloning and overexpression of the AatII restriction endonuclease and methylase and the cloning and overexpression of the AluI restriction endonuclease and methylase.