Attenuation in Bacterial Operons

Abstract

INTRODUCTION
Expression of bacterial genes and operons is often controlled by molecular events that regulate the frequencies of transcription initiation and termination at specific promoter and terminator sites. Transcriptional repressors and activators are well-known regulatory molecules that determine the frequency of transcription initiation. Repressors reduce initiation by preventing RNA polymerase binding to the promoter, whereas activators stimulate transcription by interacting with DNA, RNA polymerase, or both. Regulation of gene expression by molecular events that act on transcription termination has become evident only more recently. Antitermination involves interactions between RNA polymerase and accessory factors that lead to a modified transcription complex that no longer recognizes transcription termination signals. In contrast, attenuation, which is the subject of this paper, is the regulation of transcription termination by molecular interactions that are believed to govern the formation of the transcription termination signal itself. The term “attenuator” is generally used to describe a site where regulation of transcription termination occurs within an operon.

Attenuation was discovered during investigations on the regulation of the histidine (his) operon of Salmonella typhimurium and the tryptophan (trp) operon of Escherichia coli. Kasai (1974) showed that the regulatory region of the his operon contains a transcriptional barrier that he described as an attenuator site. He found that a small deletion (hisO1242) between the promoter and the first gene allowed greatly enhanced transcription of the structural genes in vitro compared with wild type. In the trp system, Yanofsky and his collaborators (Jackson and Yanofsky 1973; Bertrand et al. 1976) isolated a...