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Transmissible Burkholderia cepacia genomovar IIIa strains bind and convert monomeric iron(III) protoporphyrin IX into the µ-oxo oligomeric form

¹ Clinical Dental Science, University of Liverpool, Liverpool L69 3GN
² Department of Medical Microbiology and Genito-Urinary Medicine, University of Liverpool, Liverpool L69 3GN
³ School of Chemical and Life Science, The University of Greenwich, Chatham Maritime Campus, Pembroke, Chatham ME4 4TB

Correspondence
John W. Smalley
josmall{at}liv.ac.uk

Burkholderia cepacia isolates of genomovar III are highly transmissible amongst patients with cystic fibrosis (CF) and express a 97 kDa putative haem-binding protein (HBP) [Smalley, J. W., Charalabous, P., Birss, A. J. & Hart, C. A. (2001). Clin Diagn Lab Immunol 8, 509–514]. An investigation of the interactions of iron(III) protoporphyrin IX with epidemic and non-epidemic strains of B. cepacia to determine the role of the above protein in haem acquisition and binding is reported herein. Spectrophotometric titrations of cell suspensions of genomovar IIIa strains BC7 and C5424 with iron(III) protoporphyrin IX, at pH 7·0, resulted in the depletion of Fe(III)PPIX.OH monomers and formation of the µ-oxo oligomeric species, [Fe(III)PPIX]₂O. Difference spectroscopy indicated a continuous conversion of the monomeric iron(III) protoporphyrin IX into µ-oxo oligomers. Incubations with Fe(III)PPIX.OH monomers at pH 6·5 also showed that cells could shift the equilibrium to generate the µ-oxo oligomeric form. Genomovar I strains ATCC 25416 and LMG 17997 were unable to mediate this conversion. SDS-PAGE of genomovar IIIa strains exposed to Fe(III)PPIX.OH at pH 6·5 followed by tetramethylbenzidine/H₂O₂ staining revealed, in addition to the 97 kDa HBP, two proteins of 77 and 149 kDa located in the outer membrane which bound Fe(III)PPIX.OH monomers. These proteins were absent from the genomovar I strains. Genomovar IIIa strains BC7 and C5424 showed increased cellular binding of [Fe(III)PPIX]₂O, and as a consequence, displayed increased catalase activities compared to cells of the genomovar I isolates. It is concluded that, in addition to the putative 97 kDa HBP, B. cepacia genomovar IIIa strains express two outer-membrane proteins which function to bind and convert Fe(III)PPIX.OH monomers into the µ-oxo oligomeric form, [Fe(III)PPIX]₂O. The ability to perform this conversion at both neutral and slightly acidic pHs may enable epidemic strains to withstand attack from neutrophil-derived H₂O₂ in the inflamed CF lung.

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