Intragenic recombination leads to pilus antigenic variation in Neisseria gonorrhoeae

The pilus of the bacterium Neisseria gonorrhoeae is a fimbriate surface structure which promotes attachment of the bacterium to host epithelial cells^1-4. 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 ⁵, reminiscent of the MAT locus in the yeast Saccharomyces cerevisiae⁶, while several other chromosomal regions contain silent (non-expressing) pilin sequences⁵. Biochemical and antigenic diversity is seen in pili from a wide variety of clinical isolates ^7-11. Pilins (pilus subunits) are composed of conserved N-terminal and variable C-terminal regions¹²; the conserved region of gonococcal pilin is also found in pilins produced by widely disparate bacteria ^13-15. 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.