Does ferredoxin I (Azotobacter) represent a novel class of DNA-binding proteins that regulate gene expression in response to cellular iron(II)?

Azotobacter vinelandii (Av) and chroococcum (Ac) ferredoxin I contain [3Fe-4S]1 + 0 and [4Fe-4S]2+1+ clusters, when isolated aerobically, which undergo one-electron redox cycles at potentials of -460 +/- 10 mV (vs SHE) at pH 8.3 and -645 +/- 10 mV, respectively. The X-ray structure of Fd I (Av) reveals that the N-terminal half of the polypeptide folds as a sandwich of beta-strands which enclose the iron-sulphur clusters. The C-terminal sequence contains an amphiphilic alpha-helix of four turns which lies on the surface of the beta-barrel. Fd I (Av) controls expression of an unknown protein of Mr approximately 18,000. Fd I (Ac) will complex iron(II) avidly above pH approximately 8.0 only when the [3Fe-4S] cluster is reduced and provided that cellular nucleic acid is bound. Fd I (Ac) rigorously purified from nucleic acid does not undergo iron(II) uptake. These facts, together with recent evidence that the interconversion process [3Fe-4S]0 + Fe2+----[4Fe-4S]2+ in the iron-responsive element binding protein (IRE-BP) of eukaryotic cells is controlling protein expression at the level of mRNA [1991, Cell 64, 4771; 1991, Nucleic Acid Res. 19, 1739] leads to the following hypothesis. Fd I is a DNA-binding protein which interacts by single alpha-helix binding in the wide groove of DNA. The binding is regulated by iron(II) levels in the cell. The 7Fe form binds to DNA and represses gene expression. Only the DNA-bound form of the 7Fe Fd I will take up iron(II), not the form free in solution. Iron(II) becomes bound when the [3Fe-4S] cluster is reduced. The 8Fe Fd I thus generated no longer binds DNA and the gene is de-repressed. Sequence comparisons and the crystal structure suggests that the two central turns of the alpha-helix are important elements of the DNA-recognition process and that residues Gln69 and Glu73, which lie on the outer surface of the helix, hydrogen-bond with specific base pairs.