Analysis of the Functional Components of the Phase Variation System

The oscillation of flagellar phenotypes of Salmonella, termed phase variation, is regulated by a genetic rearrangement. The concept that specific recombinational processes may determine the nature of gene expression was elaborated by McClintock on the basis of her work with maize. McClintock (1957) suggested that mobile elements could become associated with specific gene systems and could be modified to regulate gene expression. The advent of new and powerful biochemical and genetic techniques has allowed the identification and physical analysis of transposable elements in prokaryotes (Kleckner 1977) and eukaryotes (Cameron et al. 1979; Potter et al. 1979). Such elements may contribute plasticity to the genome, which is important in regulating gene activity or in stimulating genetic diversity. Interconversion of yeast mating types (Strathern et al. 1980), generation of immunoglobulin diversity in mice (Maki et al. 1980), and control of antigenic variability in trypanosomes (Williams et al. 1979) all involve rearrangement of...