The pilus of the bacterium Neisseria gonorrhoeae is a fimbriate surface structure which promotes attachment of the bacterium to host epithelial cells. Gonococcal pilus phase variation is characterized by a rapid on/off switch in which piliated (P+) cells throw off non-piliated (P-) variants and vice versa. Two regions of the gonococcal chromosome (pilE1 and pilE2) act as pilin expression loci, reminiscent of the MAT locus in the yeast Saccharomyces cerevisiae, while several other chromosomal regions contain silent (non-expressing) pilin sequences. Biochemical and antigenic diversity is seen in pili from a wide variety of clinical isolates. Pilins (pilus subunits) are composed of conserved N-terminal and variable C-terminal regions; the conserved region of gonococcal pilin is also found in pilins produced by widely disparate bacteria. We show here that the gonococcal pilin undergoes antigenic variation in vitro and in vivo. The protein consists of constant, semi-variable and hypervariable regions. This antigenic variation probably involves gene conversion of mini-cassettes of pilin information.

We have mapped two regions of the Neisseria gonorrheae genome, pilEI and pilE2, which are involved in pilus expression. When the cells are in the piliated P+ state, these two loci carry sequences necessary for pilin production. A silent locus, piSI, also maps near pilEI and pUE2. pilSI contains structural gene information but lacks pilus promoter sequences. The pilus gene sequences in pilEI and piLE2 are identical in strain MS1l.

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