Insertion mutations in pilE differentially alter gonococcal pilin antigenic variation

Pilus antigenic variation in Neisseria gonorrhoeae occurs by the high-frequency, unidirectional transfer of DNA sequences from one of several silent pilin loci (pilS) into the expressed pilin gene (pilE), resulting in a change in the primary pilin protein sequence. Previously, we investigated the effects of large or small heterologous insertions in conserved and variable portions of a pilS copy on antigenic variation. We observed differential effects on pilin recombination by the various insertions, and the severity of the defect correlated with the disruption or displacement of a conserved pilin DNA sequence called cys2. In this study, we show that disruption or displacement of the pilE cys2 sequence by the same insertions or a deletion also affects pilin recombination. However, in contrast to the insertions in pilS, the analogous insertions in pilE impaired, but did not block, recombination of the flanking pilin sequences. These results, the change in the spectrum of donor silent copies used during variation, and our previous results with pilS mutations show that the donor pilS and recipient pilE play different roles in antigenic variation. We conclude that when high-frequency recombination mechanisms are blocked, alternative mechanisms are operative.