The zinc ion in the HNH motif of colicin E7 plays a catalytic role
Meng-Jiun Sui (隋孟君), Li-Chu Tsai (蔡麗珠), Yi-Sheng Chen (鄭貽生), Ludmila Doudeva (杜麗沙) and Hanna S. Yuan (袁小琀)
Institute of Molecular Biology, Academia Sinica, Taipei, Taiwan, ROC.
The colicin E7 (ColE7) from Escherichia coli is a plasmid-encoded bacteriocin, which contains endonuclease activity capable of hydrolyzing DNA chromosome in target cells. A HNH motif of approximately 30 amino acids has been identified in the DNase domain of ColE7. The crystal structure of the DNase domain of ColE7 (DNase-ColE7) in complex with Im7 reveals a Zn2+ ion located in the center of the HNH motif. In order to investigate the role of the zinc ion in ColE7, we determined the crystal structure of the DNase-ColE7/Im7 complex in the presence of different bivalent metal ions.
X-ray diffraction data were collected using synchrotron radiation at SRRC in Hisn-Chu for the Mn2+ and Zn2+ containing complex at the manganese and zinc absorption edges, respectively. The positions of Mn2+ and Zn2+ were verified in the anomalous Fourier maps. Our studies show that the manganese ion replaces the zinc ion and occupies at the same position. A phosphate ion is bound to either the zinc or manganese ion. The gel-retardation assay further showed that the DNase-ColE7 hydrolyzes DNA in the presence of zinc ions, but binds to the un-cleaved DNA without the presence of zinc ions. These results suggest that the zinc ion in the HNH motif of ColE7 plays a catalytic role and it is likely responsible for DNA binding.