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The mechanism by which iron-sulfur clusters are assembled in vivo is poorly understood. 1 The inherent toxicity of free iron and sulfide has led to the proposal that certain proteins participate in the delivery of Fe and S2-during cluster assembly. In Azotobacter Vinelandii (A. V.) and other diazotrophic bacteria, cotranscribed nifS and nifU genes of the nif-specific gene cluster are essential for the maturation of the Fe-S clusters of nitrogenase component proteins. 2 NifS catalyzes the desulfurization of cysteine to provide S2-for incorporation into Fe-S clusters. 3 NifU is a cysteine-rich, modular protein, containing a [2Fe-2S] 2+ cluster coordinated to four cysteine residues in the protein’s central domain. 4 The N-terminal domain of NifU contains three cysteine residues in the sequence CX25CX44C that do not have any similarity to known Fe-S containing proteins; however, recent studies have demonstrated a role for these …