The haemin storage (Hms) phenotype of has been shown to be involved in the blockage of fleas that is required for the transmission of plague from fleas to mammals. Previously, an operon encoding four genes, , that are essential for the temperature-regulated Hms phenotype has been characterized. Here the isolation and characterization of a fifth gene, , that is essential for this phenotype is described. Conceptual translation of suggests it encodes a 44.8 kDa protein with a pl of 7.75. The gene for HmsT is located outside of the ~ 102 kb locus of that contains the operon. Hybridization studies indicate that but not or possesses a highly homologous gene. HmsT belongs to a family of PleD-related proteins with four highly conserved regions of homology. Although PleD is a regulator, the functions of the other members of this family have not been experimentally determined. The iron-responsive regulator, Fur, has previously been implicated in temperature regulation of the Hms phenotype. A good potential Fur-binding site (FBS) is located upstream of M23 and two of five strains, which all exhibit a temperature-constitutive Hms phenotype, contain a 6 bp insertion in the putative FBS. MG1655 contains homologues of ) but has an Hms phenotype. Only and complement mutations in their respective homologues, and , in


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