Fur (ferric uptake regulation) protein and CAP (catabolite-activator protein) modulate transcription of fur gene in Escherichia coli.

A fusion between the fur (ferric uptake regulation) gene, known to mediate negative regulation of iron absorption in Escherichia coli, and lacZ was constructed in vitro. beta-Galactosidase levels of cells harboring this fusion were under the control of sequences contained in a 185-bp DNA fragment located upstream of the fur structural gene. The fusion was prepared in multicopy (pVLN102 plasmid) and low-copy-number states, the latter constructed as a lambda phage lysogen carrying a fur'-'lacZ insert. DNase I footprinting experiments with purified Fur protein, performed on a 250-bp restriction fragment carrying the promoter region of the fusion, showed the presence of a single Fur-protected site overlapping the -10 region of a potential promoter sequence. Examination of the DNA sequences located upstream of the fur gene revealed a possible binding site for the catabolite-activator protein (CAP). beta-Galactosidase synthesis of E. coli cells harboring the fusion were measured in fur, crp and cya genetic backgrounds and compared with the corresponding levels in wild-type strains. The data obtained indicate a moderate autoregulation of fur expression by its gene product and also a significant stimulation by the cAMP-CAP system. Transcription start sites were mapped by primer-extension experiments with total RNA obtained in vivo from cells harboring pVLN102. The results show that transcription of the fur gene is initiated from at least two different sites separated by 6 bp, which appear to originate from two overlapping promoters sensitive to catabolic activation.