Bacteria, such as Escherichia coli, have enzymes that require nickel ions, and they express a nickel transporter to acquire the metal from the environment. To keep cellular concentrations of nickel relatively constant, the expression of the transporter is regulated by a repressor, NikR, which is itself a nickel sensor. NikR binds to the operator of the transporter gene and represses transcription only when it is in the nickel-bound form. Schreiter et al solved the crystal structures of the nickel-bound form of NikR from E. coli both alone and in a complex with a DNA fragment corresponding to the promoter of the nickel transporter gene. The protein has two DNA-binding domains that interact with sites in the palindromic operator on either side of a metal-binding domain. In other ligand-regulated transcription factors, activation is proposed to occur when a change in the spacing between the DNA-binding domains is altered in such a way that they interact more effectively with the promoter DNA. Comparison of the new structures with the previously reported structure of nickel-free protein indicates that this is not how NikR works. Rather, it appears to create a new interactive surface within the metal-binding domain that enhances the interaction of the protein with the promoter DNA helix. The results provide a detailed look at the precise molecular changes that underlie transcriptional control by a ligand-regulated transcription factor. -- LBR
Proc. Natl. Acad. Sci. U.S.A. 103, 13676 (2006).
