Additionally, the predicted heme/hemoglobin receptors of V. splendidus (CAV26466) and V. fischeri (ACH65716) lack the histidine residue corresponding to His-461, whereas the heme receptors of V. parahaemolyticus (BAC62225), V. harveyi (ABU73683), V. anguillarum (HuvA), V. cholerae

(HutA), and V. vulnificus (HupA) possess the corresponding residues (Fig. 4). These data suggest that the mechanism for heme-binding may be somewhat different among different heme/hemoglobin receptors (3). Similarly to other bacterial heme/hemoglobin receptors (38), the manner in which the heme ligand is released from hemoglobin on its cell surface receptor in V. mimicus remains to be clarified. It was found that MhuA shows only 34% identity to V. cholerae VCA0576 (HutA) (Table 2), although MhuB and the partial amino acid sequences deduced from orf1 and orf4, genes in close vicinity to the mhu loucus, show more than HIF-1 activation 85% homology to the corresponding V. cholerae proteins, VCA0575, VCA0574, and VCA0578,

respectively. This learn more implies that the origin of mhuA is different from that of V. cholerae hutA. To further examine the evolutional relationship of the characterized and putative heme/hemoglobin receptors in Vibrio species, we constructed a phylogenetic tree (Fig. 8). The receptors can be classified into two major branches according to the presence or absence of a conserved histidine residue. V. mimicus MhuA forms a clade very distinct from the V. cholerae HutA, although these species are genomically similar to each other (7, 40). MhuB is probably a transcriptional regulator for mhuA belonging to the LysR family. Most LysR regulators repress their own transcription by binding the respective promoter regions, possibly to self-maintain them at their appropriate levels within cells (30, 41). This is consistent with the finding on RT-qPCR that only

very weak transcription of the mhuB gene occurs. Additionally, it has been reported that this type of Methocarbamol regulator usually upregulates transcription of its target genes 6- to 200-fold (29). However, since MhuB activated the mhuA transcription only about 2-fold (1.6-fold in RT-qPCR, and 2.3-fold in β-galactosidase reporter assay), it may be a weak activator of mhuA (31). On the other hand, the fate of heme internalized into the bacterial cytosol is poorly understood. Although some Gram-negative bacteria have been reported to use heme oxygenase-like enzymes (3), no heme oxygenase activity has been identified to date in Vibrio species (23, 38). Wyckoff et al. have reported that the V. cholerae HutZ, which shows no heme oxygenase-like enzyme activity but can bind heme, is required for efficient heme-iron utilization (23). In this context, a more recent article reporting that E.