The Mechanism of Phase Variation

Abstract

INTRODUCTION
Studies of the properties of the lac operon have formed the basis for our understanding of a number of regulatory mechanisms. The lac operon model focuses attention on systems that involve the binding of repressor or activator proteins to sites adjacent to structural genes, leading to enhanced expression or to repression of gene activity. There are, however, many observations involving the regulation of gene activity which cannot be explained by mechanisms analogous to the lac operon. For example in yeast, the interconversion of mating types (Hicks et al. 1977) is best explained by a model involving site-specific recombination, i.e., gene expression requires the transposition of genetic material from one chromosomal locus to another. In the Zea mays genetic system, McClintock (1957) recognized and identified transposable chromosomal elements that control the functions of a variety of genes. Many other observations in genetic systems as varied as Drosophila (Green 1977), Paramecium (Sonnenborn 1977), and Escherichia coli (Starlinger and Saedler 1972) are most readily explained by invoking a specific recombination event, e.g., an inversion or transposition, or some other kind of modification that changes the state of the gene and thus affects its expression. However, very little is known about the exact molecular events that occur in any of these systems.

Just as the development of genetic systems in E. coli allowed an analysis of the mechanism of regulation of the lac operon, so the introduction of gene cloning techniques allows a direct examination of DNA that might be involved in site-specific...