Abstract

Protein 1 (PI) is a major porin of Neisseria gonorrhoeae and Neisseria meningitidis and is encoded by a single locus, porB. Alleles of the porB locus of N. gonorrhoeae are assigned to two homology groups, PI(A) and PI(B), on the basis of immunological and structural similarity. In a like manner, alleles of the porB locus of the closely related bacterium, N. meningitidis, are allocated into class 2 and class 3 homology groups. An individual strain of N. gonorrhoeae or N. meningitidis expresses either one or other of these porin homology groups but never both, and the antigenic reactions of these highly diverse outer membrane proteins form part of the N. gonorrhoeae and N. meningitidis serotyping schemes. A comparison of the number of synonymous and nonsynonymous substitutions per site between the two most divergent alleles of each of these four groups of porB alleles shows that PI(A) alleles have accumulated significantly more nonsynonymous substitutions per site than synonymous substitutions. In contrast the distribution of synonymous and nonsynonymous substitutions between alleles of class 2 and class 3 porins are not significantly different from random. We localize the regions of the PI(A) alleles with an excess of amino acid changes to the surface-exposed loops of these outer membrane proteins and suggest that positive Darwinian selection for diversity, driven by the human immune system, can most easily explain the allelic polymorphism and the pattern of synonymous and nonsynonymous substitutions.