P F Sparling

Antigenic and Sequence Diversity in Gonococcal Transferrin-Binding Protein A

ABSTRACT Neisseria gonorrhoeae is a gram-negative pathogen that is capable of satisfying its iron requirement with human iron-binding proteins such as transferrin and lactoferrin. Transferrin-iron utilization involves specific binding of human transferrin at the cell surface to what is believed to be a complex of two iron-regulated, transferrin-binding proteins, TbpA and TbpB. The genes encoding these proteins have been cloned and sequenced from a number of pathogenic, gram-negative bacteria. In the current study, we sequenced four additional tbpA genes from other N. gonorrhoeaestrains to begin to assess the sequence diversity among gonococci. We compared these sequences to those from other pathogenic bacteria to identify conserved regions that might be important for the structure and function of these receptors. We generated polyclonal mouse sera against synthetic peptides deduced from the TbpA sequence from gonococcal strain FA19. Most of these synthetic peptides were predicted to correspond to surface-exposed regions of TbpA. We found that, while most reacted with denatured TbpA in Western blots, only one antipeptide serum reacted with native TbpA in the context of intact gonococci, consistent with surface exposure of the peptide to which this serum was raised. In addition, we evaluated a panel of gonococcal strains for antigenic diversity using these antipeptide sera.

Gene transfer in Neisseria gonorrhoeae.

The ability of bacteria to exchange deoxyribonucleic acid (DNA) endows these organisms with greater genetic variability and increased capability to adapt to changing environments. Many bacteria have evolved transformation and conjugation systems to effect this exchange, whereas others achieve it through the action of bacteriophages. In Neisseria gonorrhoeae no transducing bacteriophages have been identified, but conjugation and transformation both occur (10). Gonococci are extremely autolytic and therefore release DNA to neighboring cells (19). Transformation with chromosomal markers has been demonstrated between strains in laboratory-grown mixed cultures (32); similar transformation probably occurs in nature. Quite recently, antigenic and phase variation of gonococcal pili has been shown to be due in large part to release of DNA from autolyzing cells, with subsequent transformation of other competent cells in the population by variant pil sequences (32a; C. Haas and T. Meyer, personal communication). Gonococcal transformation has been studied extensively and has proven useful in the construction of isogenic strains for mapping antibiotic resistance genes and biosynthetic auxotrophs, and in studies of molecules implicated in the virulence of this organism (10, 34). Conjugation in N. gonorrhoeae is important because it results in mobilization of antibiotic resistance plasmids, but chromosomal genes cannot be transferred by conjugation (3, 10, 27, 31, 37, 44J. Thus, in the laboratory and in nature, transformation is the primary means of transfer of chromosomal genes. In this brief review, mechanisms of gene exchange will be emphasized, and some speculative comments on possible future developments are included.