Lac Z and Trp Operon

Lac Z and Trp Operon

Introduction: ( al., 2008)

Prokaryotes, such as E.coli, have an effective role in lactose metabolism. There are three important proteins responsible for lactose metabolism encoded in single expressible unit of DNA known as lac operon.

When the lactose is not presented in the media, the bacterium does not expend the energy made from the expression of these proteins. However, when the lactose is accessible to be metabolized the proteins will be made.

Description of the lac operon:

The lac repressor protein encoded by the I genes is expressed in the absence or presence of the lactose. In the absence of the lactose the lac repressor binds to the operator site, repressor bonded to the operator blocks progression of the RNA polymerase. Since RNA polymerase is unable to transcribe the lac structural genes, the protein will not be made.

When the lactose is presented in the cell media, it will bind to the active site of the lac repressor, this changes the confirmation of the repressor, in this confirmation the repressor can no longer bind to the lac operator site. However, without the repressor blocking its way, RNA polymerase is able to transcribe the structural genes. Thus, in the presence of lactose the structural genes are expressed, the proteins encoded by Z and X genes are required for the metabolism of the lactose.


The figure above shows how the genes are transcribed by RNA polymerase in the absence and presence of lactose.


E,coli, can grow in a variety of energy sources-containing media. In order, to use lactose, the bacterium must stop its metabolism temporarily and must switch on several genes that presented in the lac operon.

When the lactose is absence, the expression of these genes found in the lac operon is impossible because the repressor binds to the lac operon and blocks the way of the RNA polymerase resulting in preventing the transcription of the remaining structural genes. However, lactose bind to the lac repressor and changes the confirmation of the repressor in the presence of lactose, so the repressor cannot bind to the operon and the RNA polymerase can pass through it completing its way and transcribing the three genes.

The trp operon: (

The amino acid Tryptophan syntheses are made by trp operon which consists of the gene for the enzymes that required for its formation. However, the substances of the anabolic enzymes are coded by the trp operon, these substances work to turn some operons off.

In condition, where the concentration of the tryptophan is high, the tryptophan product is not needed and it will also posses an extra level of operate that not found in the lac operon.

The repressor: if the tryptophan is found in the media the trp repressor will bind it in the operator site which is an 18 bp palindrome. Therefore, the repressor is made of two subunits some times called dimer, when the repressor is combined with the tryptophan it will bind to the operator.

The structural genes: trpE, trpP, trpC, trpB and trpA these are the structural genes of the trp operon which codes for a single transcription unit built up of 7 kb in length.




1. Savada. Et al., life: the Science of biology, Eighth Edition, Sinauer Associates 2008.

Enter net:

1. Web Page: Regulation of gene expression in Prokaryotes. URL: expression/trp-operon.htm

2. Web page: The tryptophan operon.

3. Web page: Introduction of the lac operon.

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