Chapter 11 Repressor and Its Action

Abstract

In 1961 Jacob and Monod elaborated their operon concept, and described the phenomenon of lysogeny as a typical manifestation of gene control by repressors. They proposed that the product of the cI gene of phage λ directly blocks early functions required for DNA synthesis. Furthermore, the same repressor required for maintenance of the lysogenic state would also confer immunity against superinfecting phages of the prophage type by turning off their genes. Induction of the prophage would follow upon inactivation of the repressor, the presumed consequence of treatment with inducing agents such as ultraviolet light.

The argument for the existence of the λ repressor was analogous to that advanced for the existence of the lac repressor (the product of the i gene of the lac operon). Genetic experiments with both lac and λ supported these assertions: (1) there is in each case a single regulatory gene, (2) the active allele of the regulatory gene turns off the genes of the corresponding operon (“negative” control), and (3) cis-dominant mutations are found that modify the molecular targets (operators) of the repressors so that the genes ordinarily under repressor control can function even in the presence of repressor. Jacob and Monod inferred that the product of the regulatory gene acts directly on an operator to repress the genes of the repressor-controlled operon. They speculated that the operators might be DNA, and that repressors might block transcription. They at first suggested, on the basis of reports that repressor synthesis proceeds in the presence of inhibitors...