Lac operon

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Francois Jacob and Jacques Monad proposed a model for an operon, which consisted of a regulator gene, an operator site consisting of a regulatory DNA sequence and one or more structural genes.

The model displayed how stimuli from the environment can promote/inhibit genetic mechanisms which control metabolic events e.g. the presence/absence of glucose and lactose in the lac operon [1]. The lac operon consists of an additional promoter, in front of the regulator gene, the role of which is to ensure the RNA Polymerase binds to the correct transcription initiator.

The repressor protein is a homotetramer and a product of the lacI gene, and will bind tightly to the operator, under the correct conditions i.e. when glucose is present and lactose absent. When the repressor is bound to the operon, the RNA polymerase is unable to unwind the DNA in order to expose the bases and hence is unable to transcribe the structural genes as there is no template for the RNA synthesis to occur. The group of structural genes act as a single transcription unit, coding for a single mRNA molecule termed a polycistronic transcript i.e. coding for multiple proteins and transcription is dependent upon the correct environmental conditions as described below:
In the presence of lactose, the lac operon is induced by allolactose, which binds to the lac repressor and a conformational change occurs, which results in a decreased affinity of the lac repressor for the lac operator and transcription of the structural genes occurs.
In the absence of glucose, cAMP accumulates (glucose metabolites prevent this build up when glucose is present), and cAMP is able to bind to a cAMP binding site on the lac operon activating the operon and promoting transcription [2].


What is the Lac operon?

The lac operon is a good example of how genes are regulated. It is an example of how genes can be regulated through the acts of an activator and/or repressor.The lac operon was studied in E. coli [3]. It contains 3 genes that are needed to produce proteins that are required to break down lactose when it is present in the cell. These 3 genes are Lac Z, Lac Y and Lac A. Each code for B- galactosidase, Permease and Transacetylase respectively [4].

Further up the genetic code from these three genes, upstream, lies the promoter sequence. RNA polymerase needs a region in which it can join the genetic code, the Promoter sequence, before it can start transcribing. RNA polymerase is required in transcription of the Lac operon [5]. Lac operon - basic.JPG

When does the Lac operon function?

The Lac operon is regulated depending on the absense or presence of certain substances. When lactose is present in the cell and glucose is absent, then the Lac operon is active and the 3 genes are transcribed to break down this lactose in the cell.
The lac operon only needs to activate the genes necessary for lactose integration when there is an absent of glucose and plentiful supply of lactose.

Negative gene regulation

The conditions inside the cell are changing all the time. So what happens when glucose is present and lactose levels are low? The Lac operon is no longer required to make the proteins to break down lactose and so its function is switched off. This is done by the use of a repressor protein [6].
Upstream of the promoter sequence there is another gene. This is the Lac I gene. The Lac I gene is transcribed to make the repressor protein which binds to the operator sequence. The repressor protein is a tetramer and binds two operators on the template strand by looping the strand.

Lac operon - with lac I and operator seq.JPG
Lac operon - with lac I and operator seq nd repressor.JPG

Once the repressor protein is bound, it stops the RNA polymerase enzyme from transcribing the genes. Effectively, it acts as a block [7].

When the conditions in the cell change, the glucose levels deplete and the lactose levels rise, the repressor has to be removed in order to transcribe the required genes. This is done by an inducer molecule. This molecule comes from lactose and is Allolactose. This binds to the repressor protein and causes it to change, a conformational change. Once it has bound, the repressor can no longer bind to the operator sequence as it did before, its affinity has changed, and so is removed. RNA polymerase can work as it is not blocked and the Lac Z, Lac Y and Lac A genes are transcribed [8].

Lac operon - allolactose.JPG

Positive Gene regulation

Sometimes promoters are not strong enough to initiate transcription on their own and so require another molecule or complex to help. In the Lac operon, this is done by the CRP – cAMP complex.
When glucose levels in the cell are low, the levels of cAMP build up [9]. This then combines with CRP and forms a complex. The complex can then join to the promoter sequence as well as the RNA polymerase and acts as a positive activator and encourages transcription [10].
When the levels of glucose increase again, the amount of cAMP synthesised is reduced and so the complex levels decrease. This therefore inhibits the Lac operon from working.


  1. Cellular and Molecular Life Sciences, Matthews KS, Swint-Kruse L, Wilson CJ, Zhan H, The Lactose Repressor System: Paradigms for Regulation, Allosteric Behaviour and Protein Folding.January 2007; 64(1):3-16
  2. Berg Jeremy.M, Tymoczko John.L, Stryer Lubert, 2007, Biochemistry, Sixth Edition, W.H.Freeman, New York, Pages 897-900
  3. Hartl, D. L. and Jones, E. W., 2009. Genetics: Analysis of genes and genomes. 7th edition. Sudbury: Jones and Bartlett Publishers pg 383
  4. (2011) "The operon" – 30th March 2010 – Available from: [Accessed 3rd January 2011]
  5. Hartl, D. L. and Jones, E. W., 2009. Genetics: Analysis of genes and genomes. 7th edition. Sudbury: Jones and Bartlett Publishers pg 386
  6. Hartl, D. L. and Jones, E. W., 2009. Genetics: Analysis of genes and genomes. 7th edition. Sudbury: Jones and Bartlett Publishers pg 384
  7. Sadava (2011) “ The Lac Operon” – 2008 – Available from: [Accessed 3rd January 2011]
  8. BioCoach Activity (2011) "The lac inducer: Allolactose" – Available from: [Accessed 3rd January 2011]
  9. Mulligan, M. E. (2002) “The lac operon: positive regulation” – Available from: [Accessed 3rd January 2011]
  10. Hartl, D. L. and Jones, E. W., 2009. Genetics: Analysis of genes and genomes. 7th edition. Sudbury: Jones and Bartlett Publishers pg 389
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