Regulation of the expression of the nickel uptake system in Vibrio cholerae by iron and heme via ferric uptake regulator (Fur)

Fur (ferric uptake regulator) is a transcription factor that regulates expression of downstream genes containing a specific Fe2+-binding sequence called the Fur box. In Vibrio cholerae, a Fur box is located upstream of the nik operon, which is responsible for nickel uptake, suggesting that its expression is regulated by Fur. However, there are no reports that Ni2+ induces expression of Fur box genes. Accordingly, we here investigated whether Ni2+ or Fe2+ binds to Fur to regulate expression of the nik operon. We found that Fur binds to the Fur box in the presence of Fe2+ with a dissociation constant (Kd) of 1.2 μM, whereas only non-specific binding was observed in the presence of Ni2+. Thus, Fur-mediated expression of the nik operon is dependent on Fe2+, but not Ni2+. Since most iron in cells exists as heme, we examined the effect of heme on the Fur box binding activity of V. cholerae Fur (VcFur). Addition of heme to the VcFur-Fur box complex induced dissociation of VcFur from the Fur box, indicating that expression of the V. cholerae nik operon is regulated by both iron and heme. Furthermore, VCA1098, a nik operon-encoded protein, bound heme with a Kd of 1.3 μM. Collectively, our results suggest that the V. cholerae nik operon is involved not only in nickel uptake but also in heme uptake, and depends on iron and heme concentrations within bacteria.