Regulation of Gene Expression in the Tryptophan Operon of Escherichia coli

Abstract

INTRODUCTION
The classic model of Jacob and Monod (1961) for induction and repression of operon expression provided a conceptual framework for integrating observations on cellular metabolism with a mechanism of regulation at the molecular level. In recent years, direct physical evidence has confirmed the essential predictions of their hypothesis for the lactose operon, as summarized elsewhere in this volume. In general, their ideas have been exceptionally valuable when applied to other systems as well. However, regulation of the lac gene cluster has proved more intricate than originally envisioned, and regulatory features have been discovered which cannot be accommodated in the simple repressor-operator model of regulation. The tryptophan (trp) operon of Escherichia coli also possesses regulatory features of several types in addition to repressor-operator interaction, and the purpose of this review is to summarize current knowledge of the molecular aspects of structure and regulation within the trp operon.

The tryptophan operon consists of five contiguous structural genes (which code for enzymes participating in the biosynthetic pathway from chorismic acid to tryptophan) and the associated control elements (see Fig. 1). The five genes are transcribed as a single polycistronic messenger RNA approximately 7000 nucleotides long. Initiation of transcription is controlled by the interaction of the tryptophan represser (the protein product of trpR) with its target site on the DNA, the operator (trpO). Represser binding at trpO competes with RNA polymerase binding at the adjacent promoter site, trpP (Squires et al. 1975). Transcription of the messenger RNA is initiated within the promoter-operator region...