Determinants for Regulation and Initiation of Expression of Tryptophan Genes

The concept of the operon involves a cluster of genes whose functioning is coordinately regulated via an operator, located at one end, which responds to repressors or inducers (Jacob and Monod, 1961). Implicit in the concept is the notion of an initiation site or region closely associated with the operator, which is necessary for expression of the associated genes. Jacob, Ullman, and Monod (1964) presented evidence for such a genetic element in the lac operon of Escherichia coli and called it a promoter. Support for the concept of a promoter comes from prior studies of the histidine operon in Salmonella typhimurium (Ames, Hartman, and Jacob, 1963). These workers found that a deletion removing the operator and part of the first structural gene results in complete nonfunction of the eight remaining, intact structural genes.