The functions of N- and C-terminal domains of the Fur repressor of Escherichia coli in promoter recognition and dimerization were studied. We investigated the ability of fusion proteins containing the N- or C-terminal domain of Fur to dimerize and to repress a Fur-regulated lacZ fusion gene. The N-terminal domain, when fused to the C-terminal domain of the repressor CI857, repressed a Fur-regulated lacZ fusion. However, the Fur-CI857 fusion was unable to complement the growth defect of an E. coli fur mutant on fumarate and succinate. The C-terminal domain of Fur, when fused to the N-terminus of CI857, repressed a lambda Pr-regulated lacZ fusion, indicating dimerization of the chimeric protein, which is a prerequisite for CI activity. Both fusion proteins were fully active under both iron-rich and iron-poor growth conditions. We conclude that the N-terminal domain of Fur is involved in recognition of the Fur-responsive promoter and the C-terminus mediates oligomerization of the repressor.