Fig 3.

Fur mediates direct activation via variable mechanisms. (A) An iron-bound Fur dimer binds to a site further upstream of −10 and −35 motifs in the promoter region and out-competes binding of a repressor protein, thus activating transcription (left panel). Without iron, Fur does not function and the repressor protein binds to the promoter region to repress gene transcription (right panel). Three examples have been reported in E. coli and N. gonorrhoeae (50, 77). (B) An iron-bound Fur dimer binds to a site overlapping the −10 and −35 motifs in the promoter region in order to recruit RNA polymerase to activate transcription (left panel). Without iron, Fur does not bind to the promoter region and RNA polymerase is not recruited to initiate transcription (right panel). This mechanism is still a hypothesized model without experimental evidence (105).