A prominent feature exhibited by Chlamydia trachomatis growing in an iron-limiting environment is a differential pattern of protein expression. In many bacteria, iron-responsive proteins are regulated at the level of transcription by a family of repressors resembling the Escherichia coli ferric uptake regulator (Fur) protein. Although the chlamydial genome sequencing project did not unveil an obvious Fur homologue, a detailed examination indicated five unassigned open reading frames (ORFs) that would encode products with limited sequence homology to Fur. In this report, each chlamydial ORF was engineered in E. coli, and recombinant proteins were examined for functional characteristics resembling Fur. A Fur-specific polyclonal antiserum revealed that the protein encoded by ORF CT296 shares antigenic cross-recognition. Moreover, this protein forms dimers in solution in a fashion analogous to E. coli Fur. Further studies confirmed that the product of ORF CT296 is able to (i) complement Fur activity in a mutant strain of E. coli; and (ii) specifically bind to a 19 bp consensus sequence found in promoters of iron-regulated genes in E. coli. We propose a designation of dcrA (divalent cation-dependent regulator A) for ORF CT296, which encodes a protein distantly related to E. coli Fur. DcrA represents the first repressor described for this obligate intracellular bacterium.
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