Trp operon structural genes

In this Trp Operon Structural Genes post we have briefly explained about Trp operon model, structure and functions of Trp operon model of E.coli.

Trp Operon Model

The amino acid synthesis in E.coli is controlled by the tryptophan operon. When tryptophan is present in the growth medium, the Trp operon model is not active; however, when adequate Trp is present, the operon’s transcription is inhibited; however, when its supply is insufficient, transcription occurs. The Trp operon model differs from the lac operon in that Trp acts directly in the repression system rather than as an inducer.

Structure

Trp operon model contains five genes encoding tryptophan biosynthesis enzymes, as well as a promoter (RNA polymerase binding site) and an operator (binding site for a repressor protein). The Trp operon’s genes are transcribed as a single mRNA.

Tryptophan

Trp Operon Structural Genes

trpE: Anthranilate synthase I

trpD: Anthranilate synthase II

trpC: N-5’-Phosphoribosyl anthranilate isomerase and Indole-3-glycerolphosphate synthase

trpB: tryptophan synthase-B sub unit

trpA: tryptophan synthase-A sub unit

Functions

Absence of tryptophan

Tryptophan

Trp operon model: Absence of tryptophan

The repressor protein is inactive in the absence of tryptophan and cannot occupy the operator (o) position of the Trp operon model. RNA polymerase can connect to the promoter region of the operon (Ptrp) and begin transcription when the operator site is unoccupied.

Presence of tryptophan

Trp operon

Trp operon model: Presence of tryptophan

When tryptophan is present, the enzymes involved in tryptophan production aren’t required, thus their expression is turned off. This is accomplished by tryptophan attaching to the repressor, activating it so that it can now connect to the operator and cease structural gene transcription.

Because the repressor protein’s binding to the operator overlaps the promoter, RNA polymerase is unable to attach to it. As a result, transcription has come to a halt. Tryptophan is thought to act as a co-repressor in this role. Because the coupled repressor suppresses transcription, this is a negative control.

Attenuation

A second mechanism, called attenuation, is also used to control expression of the trp operon. The 5′ end of the polycistronic mRNA transcribed from the Trp operon model has a leader sequence upstream of the coding region of the trpE structural gene.

This leader sequence encodes a 14 amino acid leader peptide containing two tryptophan residues. The function of the leader sequence is to fine tune expression of the Trp operon model based on the availability of tryptophan inside the cell.

Trp operon

Trp operon model: Attenuation

The leader sequence contains four regions (numbered 1–4) that can form a variety of base paired stem-loop (‘hairpin’) secondary structures. The regions are: Region 1, region 2, region 3 and Region 4. Region 3 is complementary to both region 2 and region 4.

If region 3 and region 4 base pair with each other, they form a loop like structure called attenuator and it function as transcriptional termination. If pairing occur between region 3 and region 2, then no such attenuator form so that transcription continues.

Attenuation depends on the fact that, in bacteria, ribosomes attach to mRNA as it is being synthesized and so translation starts even before transcription of the whole mRNA is complete.

High tryptophan

Trp operon model: High tryptophan

When high levels of trp are present, the repressor protein trpR binds the operator of the trp operon, preventing continued expression of trp-synthesizing enzymes. However, trpR requires the ligand tryptophan, the product of the enzymes encoded by the operon, in order to bind the operator.

It cannot bind the operator in the absence of Trp, thereby allowing continued expression of the Trp operon model when the amino acid is needed. As Trp levels increase, Trp binds to TrpR, causing a conformational change that allows binding to the operator and repression of gene expression.

Trp therefore acts as a self-governor by regulating its own production through a negative feedback loop. Mutations that disrupt the TrpR gene lead to elevated production of Trp, even in the presence of Trp, thus reinforcing the notion that negative feedback on the Trp operon model is TrpR-dependent

Low tryptophan

Trp operon model: Low tryptophan

Charged tRNAs are not available for ribosome that is translating the nascent 5’end of Trp operon model mRNA.

 Hence the ribosome stalls for a while at region 1, making region 2 available to base pair with region 3 as it is transcribed. When this happens terminator loop of region 3:4 cannot form allowing the polymerase to move past attenuator to transcribe structural genes for tryptophan biosynthesis.

Hence the availability of tryptophan controls whether transcription of this operon will stop early (attenuation) or continue to synthesize a complete polycistronic mRNA.

Regulation

Trp is synthesized in 5 steps each required a particular enzyme.in E.coli chromosome the genes encoding these enzymes are located adjacent to one another in the same order as they are used in the biosynthetic pathway they are translated from a single polycistronic mRNA molecule.

These genes are called TrpE, TrpP, TrpC, TrpB, TrpA, The TrpE gene is the first one translated. Adjacent to the Trp E gene are the promoter, the operator &2 region, called the leader and the attenuated which are designated as TrpL & TrpA respectively.

The repressor gene TrpR is located quite for from the gene cluster. The regulatory protein of the repressor system o the TrpR operon is the product of the TrpR gene. Mutations either in this gene or in the operator cause constitute initiation of transcription of Trp-mRNA on the lac operon.

This regulatory protein is called Trp apo repressor &it does not bind to the operator, unless Trp is present. The apo repressor & the tryptophan molecule joins together to form an active trp repressor which binds to the operator.

Further Readings

Reference