TrpR gene

Schematic diagram of the repressor control of trp operon expression. The trp promoter (P) and trp operator (O) regions overlap. The trp aporepressor is encoded by a distantly located trpR gene. L-Tryptophan binding converts the aporepressor to the repressor that binds at the operator locus. This complex prevents the formation of the polymerase-promoter complex and transcription of the operon that begins in the leader region (trpL). Only a fraction of the transcripts extends beyond the attenuator locus in the leader region. The regulation of this fraction is discussed in the text.

The product of the trpR gene is an aporepressor protein consisting of four identical subunits, which does not bind to the operator unless tryptophan is present. A complex of the aporepressor and tryptophan binds tightly to the operator and prevents the RNA polymerase from binding to the trp promoter, so that transcription is prevented. The reactions are as follows ... 

The trp operon has two levels of regulation—an on-off mechanism and a modulation system. The protein product of the trpR gene—the trp aporepressor—cannot bind to the operator in contrast with the lac repressor. However, if tryptophan is present, the aporepressor and the tryptophan molecule join together to form an active repressor complex... 

Hiraga et al. [214] found the trpS gene mapped close to the streptomycin resistance locus, str, at some distance from both the trpR gene and the tryptophan operon. Subsequently it was reported [187,188, 215] that the trpS gene was cotransducible with and located between the str and malA loci. 

Gunsalus, R. P. and Yanofsky, C., Nucleotide sequence and expression of Escherichia coli TrpR, the structural gene for the trp aporepressor, Proc. Natl. Acad. Sci., USA, 77, 7117, 1980. 

Ito and Crawford [132] studied repression control by tryptophan of the enzyme products of the five tryptophan genes in E. coli. They found that synthesis of all the enzymes assayed (AS, PRT, InGPS, TS-B, and TS-A) was repressed in the presence of tryptophan and that removal of tryptophan resulted in coordinate derepression. In addition they confirmed the finding of Cohen and Jacob [15] that constitutive tryptophan enzyme synthesis resulted from mutation of a regulator locus (currently called trpR) unlinked to the tryptophan genes, but linked to a threonine (thr) locus. Bauerle and Margolin [133], studying the tryptophan enzymes in S. typhimurium, found what they referred to as semicoordinate... 

Morse, D. E., Yanofsky, C. Amber mutants of the trpR regulatory gene. J. molec. Biol. 44, 185-193 (1969). 

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