Trp Operon

A brief description

Introduction

• a repressible system

• In this system, though, unlike the lac operon, the gene for the repressor is not adjacent to the promoter

• but rather is located in another part of the E. coli genome. Another difference is that the operator resides entirely within the promoter

Introduction

• The TRP (Trptryptophan) operon has three components:

1. Five Structural Genes: – These genes contain the genetic code for the

five enzymes in the Trp synthesis pathway

Introduction

2. One Promoter: – DNA segment where RNA polymerase binds

and starts transcription

3. One Operator: – DNA segment found between the promoter

and structural genes. It determines if transcription will take place. If the operator is turned "on", transcription will occur.

system

• When nothing is bonded to the operator, the operon is "on".

• RNA polymerase binds to the promoter and transcription is initiated.

system

• The five structural genes are transcribed to one mRNA strand.

• The mRNA will then be translated into the enzymes that control the Trp synthesis pathway.

system

• The operon is turned "off" by a specific protein called the repressor.

• The repressor is a product of the regulator gene which is found some distance from the operon. – Transcription of the regulator produces mRNA

which is translated into the repressor. – The repressor is inactive in this form and can

not bind properly to the operator with this conformation.

system

• To become active and bind properly to he operator, a co-repressor must associate with the repressor. – The co--repressor for this system is Trp

• This makes sense because E. coli does not want to synthesize Trp if it is available from the environment

• The more Trp available, the more that can associate with repressor molecules.

system

• An active repressor binds to the operator blocking the attachment of RNA polymerase to the promoter.

• Without RNA polymerase, transcription and translation of the structural genes can't occur and the enzymes needed for Trp synthesis are not made.

facts

• Facts about the Trp operon:

1. It is essential in providing the amino acid tryptophan

2. Although the repressor protein is made continuously it cannot bind to the operator of the gene by itself.

3. If the cell is fed the amino acid tryptophan, the operon shuts down

facts

4. The product of the trp operon, tryptophan, combines with the repressor protein to from a complex which coats the operator, blocking the action of RNA polymerase.

5. This feedback inhibition conserves energy, allowing the cell to use the glucose (ATP) for other purposes.