A Hemerythrin-like Domain in a Bacterial Chemotaxis Protein

Abstract

Hemerythrin (Hr) is an O₂-carrying protein found in some marine invertebrates. A conserved sequence motif in all Hrs provides five histidine and two carboxylate ligands to an oxo-/hydroxo-bridged diiron active site, as well as a hydrophobic O₂ binding pocket. Database searches located a previously unrecognized Hr-like sequence motif at the 3‘ end of the gene, dcrH, from the anaerobic sulfate-reducing bacterium, Desulfovibrio (D.) vulgaris (Hildenborough). This gene encodes a putative methyl-accepting chemotaxis protein, DcrH. We have established by immunoblotting that a full-length DcrH, including the Hr-like domain, is expressed in D. vulgaris (Hildenborough). The C-terminal domain of DcrH, when expressed separately in recombinant form in Escherichia coli, was found to fold into a stable protein, DcrH-Hr. The UV−vis absorption and resonance Raman spectra of DcrH-Hr, and of its azide adduct, provide clear evidence for an oxo-bridged diiron(III) site very similar to that found in Hr. Based on UV−vis absorption spectra, exposure of the reduced (colorless, presumably diferrous) DcrH-Hr to air resulted in formation of an O₂ adduct also very similar to that of Hr. Unlike that of Hr, the O₂ adduct of DcrH-Hr autoxidized within a few minutes at room temperature. The O₂ binding pocket of DcrH-Hr appears to be larger than that of Hr. Given the air-sensitive nature of D. vulgaris and the putative chemotactic function of DcrH, one possible role for the Hr-like domain of DcrH is O₂-sensing. DcrH-Hr is the first characterized example of a Hr-like protein from any microorganism.