Variation within serovars of Neisseria gonorrhoeae detected by structural analysis of outer-membrane protein PIB and by pulsed-field gel electrophoresis

Outer-membrane protein PI is the antigen responsible for serovar specificity of Neisseria gonorrhoeae and is a potential vaccine target. In order to investigate possible hidden variation within a serovar, the sequence of the por genes encoding protein PIE have been obtained from a series of strains, including isolates known to be epidemiologically linked. The inferred amino acid sequences of the PIE molecules of isolates from known sexual contacts were identical, but non-related isolates showed significant heterogeneity in PIE sequence. These differences were not confined to the two variable regions (Var1 and Var2) which have previously been identified, but were largely, although not exclusively, located in regions predicted to form one of eight surface-exposed loops. The isolates were subjected to pulsed-field gel electrophoresis of restriction digests of chromosomal DNA, which also demonstrated identity between linked strains but revealed diversity within a serovar. The deduced amino acid sequences of PIE were also used to synthesize peptides for epitope-mapping experiments. These revealed that some mAbs, used to define serovar specificity, recognized linear epitopes located in loops 5 and 6, while others appeared to recognize conformational epitopes elsewhere in the molecule. The occurrence of the sequence differences within a serovar, which are not detected by the serotyping reagents, reveals that PIE represents a potential source of information which should permit considerably more detailed epidemiological studies than are currently possible and focuses attention on more conserved regions of the protein as potential targets for vaccination.