Defense Date


Document Type


Degree Name

Doctor of Philosophy


Microbiology & Immunology

First Advisor

Cynthia Cornelissen


Neisseria gonorrhoeae is an obligate human pathogen that requires iron for its survival within the host. N. gonorrhoeae expresses high-affinity iron acquisition systems to acquire iron 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, while TbpB is a surface-exposed lipoprotein. Unlike TbpA, TbpB is not required for transferrin utilization, but makes the process more efficient. The precise mechanism by which TbpA and TbpB function to mediate transferrin-iron uptake has not been fully characterized. However, the mechanism of iron acquisition from transferrin is distinct from characterized TonB-dependent ferric-siderophore uptake systems. The transferrin-iron uptake system is unique in two ways: the involvement of the TbpB lipoprotein component and the process of iron acquisition and internalization. Unlike siderophore transporters, the transferrin-iron uptake system requires the removal of iron from transferrin for its subsequent internalization. Based on analogy with characterized TonB-dependent transporters, TbpA is proposed to consist of two distinct domains: a b-barrel and plug domain. Previous studies suggest that the plug domain has a specific role in iron internalization and this study addresses the role of the plug domain in transferrin-iron acquisition. It is thought that the TbpA plug domain facilitates iron removal from transferrin and subsequent iron binding and transport. To analyze this, iron binding by the TbpA plug domain was performed and site-directed substitution mutagenesis of putative iron-coordinating residues was carried out. From these analyses, it can be concluded that the plug domain binds iron and likely plays an active role in the process of iron internalization. Mutagenesis revealed specific residues of the plug domain critical for transferrin-iron uptake, but defects imparted by these mutations were compensated for by TbpB. Thus, this study also attempts to characterize the compensatory function provided by TbpB. Through mutagenesis, critical domains involved in the efficiency of transferrin-iron acquisition were identified. One additional study describes and characterizes a novel mechanism of TonB-independent transferrin-iron acquisition. Overall, these studies further elucidate mechanisms utilized by Neisseria gonorrhoeae in the process of iron acquisition from human transferrin.


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VCU Theses and Dissertations

Date of Submission

September 2008