Domain organization and metal ion requirement of the Type IIS restriction endonuclease MnlI

A two-domain structure of the Type IIS restriction endonuclease MnlI has been identified by limited proteolysis. An N-terminal domain of the enzyme mediates the sequence-specific interaction with DNA, whereas a monomeric C-terminal domain resembles bacterial colicin nucleases in its requirement for alkaline earth as well as transition metal ions for double- and single-stranded DNA cleavage activities. The results indicate that the fusion of the non-specific HNH-type nuclease to the DNA binding domain had transformed MnlI into a Mg2+-, Ni2+-, Co2+-, Mn2+-, Zn2+-, Ca2+-dependent sequence-specific enzyme. Nevertheless, MnlI retains a residual single-stranded DNA cleavage activity controlled by its C-terminal colicin-like nuclease domain.