FIG. 3.

Amino acid comparisons of the conserved domains of the heme and Hb receptors. (A) A highly conserved receptor domain containing an invariant histidine residue, FRAP and NPNL amino acid boxes, present in all receptors that transport heme into the periplasm. Siderophore, vitamin B₁₂, and some Tf and Lf receptors lack either the complete domain or have only the FRAP box and distal glutamic amino acid residues relatively well conserved. Highly conserved residues (indicated by asterisks), aromatic residue (Aro), and gaps introduced to maximize alignment (indicated by dashes) are shown. Hpt, haptoglobin. (B) Amino acid comparison of Hb and heme receptor domains around conserved histidine residues (conserved histidine residues are shown in bold). Y. enterocolitica HemR (HEMR), Y. pestis HmuR (HMUR), E. coli ChuA (CHUA), S. dysenteriae SduA (SHUA), H. influenzae HxuC (HXUC), E. coli FepA (FEPA), and E. coli FhuA (FHUA) are shown. Amino acid residues of all receptors except FepA and FhuA are numbered from the first methionine.