Mechanisms of Pilus Antigenic Variation inNeisseria gonorrhoeae

The alteration of gene expression by DNA rearrangement appears to be an essential part of most biological systems. When an extremely high level of diversification for a set of gene products with a common function is advantageous, these rearrangements are achieved by the reassortment and recombination of repeated gene segments. The generation of the variable domains in avian immunoglobulins, antigenic variation of surface proteins in African trypanosomes and Borrelia hermsii, and pilus antigenic variation in Neisseria gonorrhoeae each results from the transfer of genetic information from nonexpressed donor alleles or pseudogenes to an active expression locus, a process likened to gene conversion occurring in spore-forming fungi. In this chapter, the author concentrates primarily on recent studies that have influenced understanding of variable pilus expression in gonococci. The vast majority of recombination events between double-stranded DNA forms of the filamentous bacteriophage f1 appear to be a consequence of gene conversion, and the mechanism appears to be the formation of asymmetric heteroduplex molecules. It may also seem that reciprocal exchange and gene conversion demand fundamentally distinct recombination reactions, but mechanistically, these two outcomes may simply be alternative resolutions of the same basic event. The identification of equivalent gonococcal genes by interspecies complementation or by virtue of their DNA homologies and the construction of mutants altered in their expression should make it possible to define their role in the chemistry of the exchange reaction.