Trp operon

The Lac operon is but one example of the genetic adaptations which allow bacteria to respond to their environment. Other examples are to be found in amino acid metabolism, for example the TRP operon which regulates tryptophan metabolism. 

The enzymes of Trp-biosynthesis are only required if too little tryptophan is available to the bacteria from the growth medium. In such a case the Trp requirement is fulfilled by the cell s own Trp biosynthesis. If however, there is enough Trp supplied by the medium, then it is prudent to shut down the Trp operon. The sensor is the Trp concentration. The Trp repressor registers the current Trp concentration with the help of its own Trp binding site. If a great deal of Trp is present, then the Trp binding site of the repressor is occupied by Trp. The Trp repressor binds Trp with high affinity (Kd=10 -10 M), upon which transcription of the operon is then blocked. 

At low Trp concentration the Trp repressor is mainly in the unbound, inactive form. The free form of the Trp repressor binds with a ca. lO -fold lower affinity to the recognition sequence than the Trp-bound form. The promoter remains free under these conditions and transcription of the genes for Trp biosynthesis can occur. The shutting on and off of the Trp operon is based on the disparate DNA affinities of the free and Trp-boimd repressor. ... 

The trp operon attenuation mechanism uses signals encoded in four sequences within a 162 nucleotide leader region at the 5 end of the mRNA, preceding the initiation codon of the first gene (Fig. 28-21a). Within the leader lies a region known as the attenuator, made up of sequences 3 and 4. These sequences base-pair to... 

FIGURE 28-19 The trp operon. This operon is regulated by two mechanisms when tryptophan levels are high, (1) the repressor (upper left) binds to its operator and (2) transcription of trp mRNA is attenuated (see Fig. 28-21). The biosynthesis of tryptophan by the enzymes encoded in the trp operon is diagrammed at the bottom... 

Transcription Attenuation How would transcription of the E. coli trp operon be affected by the following manipulations of the leader region of the trp mRNA ... 

Attenuation. A major mechanism of feedback repression, known as attenuation, depends not upon a repressor protein but upon control of premature termination. It was first worked out in detail by Yanofsky et al. for the trp operon of E. coli and related bacteria.184 186 Accumulation of tryptophan in the cell represses the trp biosynthetic operon by the action of accumulating tryptophanyl-tRNATlP, which specifically induces termination in the trp operon. Other specific "charged" arnino-acyl-tRNA molecules induce termination at other amino acid synthesis operons. 

Enzymes That Catalyze Amino Acid Biosynthesis Are Regulated at the Level of Transcription Initiation The trp Operon Is Also Regulated after the Initiation Point for Transcription Genes for Ribosomes Are Coordinately Regulated Control of rRNA and tRNA Synthesis by the rel Gene... 

The trp Operon Is Also Regulated after the Initiation Point for Transcription... 

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.

Other experiments indicated that the fraction of tRNA that is charged with an amino acid is a crucial factor in the attenuation response. This has been examined in vivo by comparing the trp operon enzyme levels in trpRT strains that are otherwise normal with strains that are defective in some respect in charged tRNATrp. Such structural defects in tRNATrp or in the charging enzyme elevates expression, probably by permitting polymerase to transcribe through the attenuator. 

Model for attenuation in the trp operon, showing ribosome and leader RNA. (a) Where no translation occurs, as when the leader AUG codon is replaced by an AUA codon, stem-and-loop 3.4 is intact, and termination in the leader is favored. (b) Cells are selectively starved for tryptophan so that the ribosome stops prematurely at the tandem trp codons. Under these conditions, stem-and-loop 2.3 can form, and this is believed to lead to the disruption of stem-and-loop 3.4. (c) All amino acids, including excess tryptophan, are present so that stem-and-loop 3.4 is present. 

The trp operon contains a cluster of five structural genes associated with tryptophan biosynthesis. Initiation of transcription of the trp operon is regulated by a repressor protein that functions similarly to the lac repressor. The main difference is that the trp repressor action is subject to control by the small-molecule effector, tryptophan. When tryptophan binds the repressor, the repressor binds to the trp operator. Thus, the effect of the small-molecule effector here is opposite to its effect on the lac operon. When tryptophan is present, there is no need for the enzymes that synthesize tryptophan. 

The trp operon has a control locus called an attenuator about 150 bases after the transcription initiation site. The attenuator is regulated by the level of charged tryptophan tRNA, so that between 10% and 90% of the elongating RNA polymerases transcribe through this site to the end of the operon. Low levels of trp tRNA encourage transcription through the attenuator. 

The trp operon contains five structural genes encoding enzymes for tryptophan biosynthesis, a trp promoter (Ptrp) and a trp operator sequence (Otrp). The operon is transcribed only when tryptophan is scarce. 

When tryptophan is lacking, a trp repressor protein (encoded by the trpR operon) is synthesized. The trp repressor dimer is inactive, cannot bind to the trp operator and so the trp operon is transcribed to produce the enzymes that then synthesize tryptophan for the cell. When tryptophan is present, tryptophan synthesis is not needed. In this situation, acting as a corepressor, tryptophan binds to the repressor and activates it so that the repressor now binds to the trp operator and stops transcription of the trp operon. 

The trp operon is regulated by both repression (which determines whether transcription will occur or not) and attenuation (which fine tunes transcription). Other operons for amino acid biosynthetic pathways may also be regulated by both repression and attenuation or only by attenuation. 

Fig. 1. Regulation of the trp operon (a) transcription in the absence of tryptophan (b) no transcription in the presence of tryptophan.

Organization of The tryptophan (trp) operon (Fig. 1) contains five structural genes encoding... 

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