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Infect. Immun. doi:10.1128/IAI.00020-08
Copyright (c) 2008, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Identification of TbpA residues required for transferrin-iron utilization by Neisseria gonorrhoeae

Department of Microbiology and Immunology, Virginia Commonwealth University Medical Center, Richmond, Virginia 23298-0678

^* To whom correspondence should be addressed. Email: cncornel{at}vcu.edu.

	Abstract

Neisseria gonorrhoeae requires iron for survival in the human host, and therefore expresses high-affinity receptors for iron acquisition from host iron-binding proteins. The gonococcal transferrin-iron uptake system is composed of two transferrin binding proteins: TbpA and TbpB. TbpA is a TonB-dependent, outer membrane transporter critical for iron acquisition, while TbpB is a surface-exposed lipoprotein that increases the efficiency of iron uptake. The precise mechanism by which TbpA mediates iron acquisition has not been elucidated; however, the process is unique from characterized siderophore transporters. Similar to these TonB-dependent transporters, TbpA is proposed to have two distinct domains: a -barrel and plug domain. We hypothesize that the TbpA plug coordinates iron and therefore potentially functions in multiple steps of transferrin-mediated iron acquisition. To test this hypothesis, we targeted a conserved motif within the TbpA plug domain and generated single, double, and triple alanine substitution mutants. Mutagenized TbpAs were expressed on the gonococcal cell surface and maintained wild-type transferrin binding affinity. Single alanine substitution mutants internalized iron at wild-type levels, while the double and triple mutants showed a significant decrease in iron uptake. Moreover, the triple alanine substitution mutant was unable to grow on transferrin as a sole iron source; however, expression of TbpB compensated for this defect. These data indicate that the conserved motif between residues 120-122 of the TbpA plug domain is critical for transferrin-iron utilization, suggesting this region plays a role in iron acquisition that is shared by both TbpA and TbpB.