16 Transcriptional Attenuation

Abstract

OVERVIEW
Transcriptional attenuation is a mechanism for gene regulation in which transcriptional termination at a specific site within an operon, called an attenuator, is controlled by a particular metabolic signal. In bacteria, there are many examples of attenuation that differ in the way that termination is made conditional. We have divided them into four classes based on their common features. A single, well-studied example of each class is described in detail, and other examples are mentioned to illustrate unique points. Possible cases of attenuation in eukaryotes are described. We also discuss the potential advantages and possible evolution of attenuation, as well as future prospects for studies in this field.

INTRODUCTION
In bacteria, transcriptional regulation can be accomplished by altering either initiation or termination of transcription. The control of gene expression by changes in the extent of termination at a site preceding one or more structural genes of an operon is called transcriptional attenuation. The interesting history of how attenuation was discovered dates to the original concept of repressors and operators, but is well documented in recent reviews (Artz and Holzschu 1983; Landick and Yanofsky 1987a) and, except for a brief account of the discovery of attenuation control of pyrimidine gene expression, is not recounted here. What has proven most fascinating about attenuation is that diverse mechanisms have evolved to control the level of many different genes by coupling transcript elongation to a wide variety of metabolic signals. We aim here to classify these mechanisms, to describe in detail the best-understood...