2Institute of Biochemistry

The bacterial toxin ColE7 contains an H–N–H endonuclease domain (nuclease ColE7) that digests cellular DNA or RNA non-specifically in target cells, leading to cell death. In the host cell, protein Im7 forms a complex with ColE7 to inhibit its nuclease activity. Here, we present the crystal structure of the unbound nuclease ColE7 at a resolution of 2.1 Å. Structural comparison between the unbound and bound nuclease ColE7 in complex with Im7, suggests that Im7 is not an allosteric inhibitor that induces backbone conformational changes in nuclease ColE7, but rather one that inhibits by blocking the substrate-binding site. There were two nuclease ColE7 molecules in the P1 unit cell in crystals and they appeared as a dimer related to each other by a non-crystallographic dyad symmetry. Gel-filtration and cross-linking experiments confirmed that nuclease ColE7 indeed formed dimers in solution and that the dimeric conformation was more favored in the presence of double-stranded DNA. Structural com-parison of nuclease ColE7 with the His-Cys box homing endonuclease I-Ppo I further demonstrated that H–N–H motifs in dimeric nuclease ColE7 were oriented in a manner very similar to that of the bba-fold of the active sites found in dimeric I-Ppo I. A mechanism for the binding of double-stranded DNA by dimeric H–N–H nuclease ColE7 is suggested