Role of tRNATrp and Leader RNA: Secondary Structure in Attenuation of the trp Operon

Abstract

Expression of several bacterial amino acid biosynthesis operons is controlled at least in part by attenuation, or transcription termination, which occurs at a site in the operon’s leader region between the promoter and the 5′ end of the first structural gene (Brenchley and Williams 1975; Bertrand and Yanofsky 1976; Zurawski et al. 1978a; Barnes 1978). Attenuation in the tryptophan (trp) biosynthetic operon of Escherichia coli has been studied mainly by Yanofsky and his colleagues. In this chapter, we review data characterizing trp operon attenuation, with emphasis on the involvement of tryptophan tRNA (tRNA^Trp) in the attenuation process.

THE ATTENUATOR IS A REGULATORY LOCUS
Yanofsky and his colleagues established the existence of the attenuator site by showing that in unstarved cells, only 10% of the RNA polymerase molecules that initiate transcription actually transcribe the structural genes. The other 90% terminate transcription about 140 bp from the promoter site. They also found that when cells are starved of tryptophan, termination of transcription at this site is relieved (Yanofsky 1976). Thus, the attenuator is a regulatory site allowing the cell to respond to the level of tryptophan (or some metabolically related compound) in its environment by allowing transcription of the entire operon or terminating transcription before the structural genes.

INVOLVEMENT OF CHARGED tRNA^Trp
Morse and Morse (1976) used trpS mutants to establish that the attenuation process is dependent not on the presence of free tryptophan, but rather on the proper functioning of tryptophanyl-tRNA synthetase in aminoacylating tRNA^Trp and, more recently, Yanofsky and Soll...