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Summary

Three iron-regulated surface determinant (Isd) proteins, containing NEAr Transporter (NEAT) domains (GBAA4789-7), constitute part of an eight-member Bacillus anthracis operon. GBAA4789 (IsdC), previously characterized by others as a haem-binding protein, and two novel Isd proteins characterized in this study, GBAA4788 (IsdJ) and GBAA4787 (IsdK) proteins, can be translated from two alternative overlapping transcriptional units. The three NEAT-containing Isd proteins are shown to be expressed in vivo during B. anthracis infection. Expression in vitro is regulated by iron ions independent of the virulence plasmids pXO1 and pXO2, yet their presence affects the range of response to iron ion concentration. The expression of IsdC, J and K is strongly repressed under high CO2 tension, conditions that are optimal for B. anthracis toxin and capsule expression, suggesting that these Isd proteins are elements of a B. anthracis‘air-regulon’. Deletion mutants of isdC, isdK or the entire isdCJK locus are as virulent and pathogenic to guinea pigs as the fully virulent wild-type Vollum strain. The isdC-deleted mutant is defective in sequestration of haemin, consistent with previous biochemical observations, while the ΔisdK mutant is defective in haemoglobin uptake. Studies with recombinant IsdK demonstrate specific binding to haemoglobin.