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The transcriptional repressor Fur (ferric uptake regulator) is one of the most important switches regulating prokaryotic iron metabolism. Cyanobacterial FurA binds heme in the micromolar concentration range and this interaction negatively affects its in vitro DNA binding ability in a concentration-dependent manner. Using site-directed mutagenesis along with difference absorption UV-visible, circular dichroism and electronic paramagnetic resonance spectroscopies, we further analyse the nature of heme binding in FurA. Our data point to Cys141, within a Cys-Pro motif, as an axial ligand of the Fe(III) high-spin heme. In the Fe(II) oxidation state, the heme shows low-spin form with an electronic absorption spectrum typical of six-coordinated low-spin heme proteins with a Soret absorption maximum blue-shifted by 25 nm in relation to typical low-spin thiolate-ligated Fe(II) heme proteins. Moreover, the ferrous C141S mutant shows Soret, a and b bands almost identical to those observed for ferrous wild-type heme-FurA, indicating that the heme in ferrous C141S is in the same six-coordinated heme ligation as the ferrous native form. Therefore, Cys141 is not a ligand of the Fe(II) heme centre, suggesting a redox-dependent ligand switch undergone by this regulator. Our results indicate that the binding of heme to FurA exhibits the same physicochemical features as previously described for heme sensor proteins.

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The DNA-binding protein from stationary phase (Dps) protein family plays an important role in protecting microorganisms from oxidative and nutritional stresses. In silico analysis of the promoter region of alr3808, a dpsA homologue from the cyanobacterium Nostoc sp. PCC7120 shows putative ironboxes with high homology with those recognized by FurA (ferric uptake regulator). Evidence for the modulation of dpsA by FurA was obtained using in vitro and in vivo approaches. SELEX linked to PCR was used to identify P dpsA as a FurA target. Concurrently, EMSA assays showed high affinity of FurA for the dpsA promoter region. DpsA expression analysis in an insertional mutant of the alr1690-afurA message (that exhibited an increased expression of FurA) showed a reduced synthesis of DpsA. These studies suggest that FurA plays a significant role in the regulation of the DpsA.

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