The role of direct oligonucleotide repeats in gonococcal pilin gene variation.

Previous studies indicate that gonococcal pilin phase and antigenic variation occur by intragenomic pilin gene recombination, the outcome of which resembles that of gene conversion. During such transitions, the expressed complete pilin gene (pilE) acquires a novel sequence corresponding to that of a silent pilin gene (pilS). In the present study, we find that internal deletions of pilE can produce pilus-/pilus+ phase transitions: direct oligonucleotide repeats in the pilin-encoding portion of pilE bracket the deleted segments. A novel, orthodox pilE is formed upon repair of the internal deletions, with pilS sequence probably acting as a template for repair. Such deletion/repair of pilE is suggested as a principal mechanism underlying gonococcal pilus variation.