QUESTIONS ABOUT GONOCOCCAL PILUS PHASE-VARIATION AND ANTIGENIC VARIATION

Pathogenic organisms inhabit one of several defined locations within a host where temperature, pH, and nutrients are relatively constant. Wh ile the microorganism must adapt to different environments within the host, the host immune system is the most formidable predator that can limit the growth of a pathogen, Neisseria gonorrhoeae (the gonococcus, Gel is the causative agent of gonorrhoea, and has evolved several sys tems for varying the antigenicity of different surface antigens, presu mably to help evade the effects of the human immune system. The On/Off /On phase variation of surface structure expression also alters the an tigenic characteristics of the bacterial cell surface. Antigenic varia tion of the major subunit of the pilus, pilin, occurs by unidirectiona l, homologous recombination between a silent locus and the expression locus. The silent loci lie from 1 to 900 kb from the expression locus in the chromosome yet all can donate their sequences to the expression locus. The genetic composition of the pilin loci of two Gc strains ha s been elucidated, and the types of changes that lead to altered forms of the pilus have been extensively characterized. However, little is known about the precise molecular mechanisms used to allow high-freque ncy, non-reciprocal, chromosomal recombination between pilin loci or a bout what regulates the process of maintaining chromosome fidelity.