Nickel is an essential cofactor for several proteins found in bacteria, but it is also toxic at high concentrations. Consequently, microorganisms need a highly regulated and specific import system of Ni2+. In Escherichia coli, this system is composed of five proteins encoded by the nikabcde operon (which is an ATP Binding Casette): NikA, a periplamic protein, NikB and NikC, two transmembrane proteins which create a pore in the inner membrane, and the cytoplasmic proteins NikD and NikE, which hydrolyse ATP and provide the energy for the transport process. FNR (Fumarate Nitrate Regulator) is the activator, and NikR the repressor of the transcription of the nik operon. We solved the NikA protein structure by molecular replacement and found that contrary to what was reported in a previous crystallographic analysis, the protein doesn't bind a pentahydrate nickel, but an FeEDTA(H2O)- complex. The EDTA, that is not a natural molecule, was introduced during the periplasmic extraction. These results suggest that NikA does not fix nickel directly, but it does it through a natural metallophore. Indeed, when we changed the periplasmic extraction with EDTA to a protocol using chloroform, we obtained a new structure where the nickel in complexed to a molecule similar to EDTA. Since this seems to be the physiological metallophore we have tried to characterise it using a variety of techniques. We also report a preliminary crystallographic analysis of NikR
