Operon system and Lac operon in E. coli

Operon system:

Bacterial structural genes are often organized into a cluster that includes genes coding for the protein whose functions are released. A cluster of the gene coding for proteins that functions in the same pathway may be located adjacent to one another and control by a single unit that is transcribed into polycistronic mRNA is called operon. Also, the operon is the coordinated unit of genetic expression in bacteria. The activity of operon is controlled by the regulatory gene. The concept of operon was introduced by Jacob and Monod in 1961 (Nobel prize 1965), based on their observations on the regulation of lactose metabolism in E.coli. This is popularly known as lac operon. One of the best examples of operon system is “lac operon in E.coli” which illustrates both positive and negative regulation.

In other words, we can say an operon is a complete unit of gene expression often involving genes encoding several polypeptides on a polycistronic mRNA. The operon system includes the two major portions, regulatory portion, and regulated portion. In bacterias the structural genes that code for the enzymes of the metabolic pathway are often found grouped together on the chromosome with the regulatory genes, that determine their transcription as a single long piece of mRNA. This entire package of genes is referred to as an operon.

Lac operon in E.coli:

Lac operon in E.coli consists of mainly two parts i.e. structural portion and regulatory portion.

Structural portion:

Structural portion codes for three enzymes involved in the catabolism and utilization of sugar lactose. The lac Z gene codes for the β-galactosidase enzyme which hydrolyzes lactose into glucose and galactose.

Lac Y gene codes for galactosidase permease that facilitates the entry of lactose into the cell.

Lac A gene codes for thiogalactoside transacetylase enzyme whose physiological function is not known yet.

Regulatory portion:

The regulatory portion of lac operon consists of lac I gene (repressor gene) that codes for the repressor protein, catabolite gene activator protein (CAP) binding site where cyclic AMP-CAP protein complex can bind, the promoter site where RNA polymerase enzyme binds and the operator site where a regulatory molecule binds.

Case 1: When only glucose is available:

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Remember: Two conditions are necessary for lac operon to be ON, i.e. to initiate transcription of structural genes:

-Operator site must be empty

-The cap-binding site must be occupied by the cAMP-CAP complex.

In this case, there is no need to synthesize the enzymes.

Mechanism:

The repressor protein synthesized by lac I gene or repressor gene binds to the operator site on the DNA which is downstream of the promoter region. The binding of repressor protein to the operator site interferes with the progress of RNA polymerase that binds to the promoter region. Hence in this case transcription of structural genes are halted, then no RNA is transcripted, i.e. lac operon remains OFF.

It is a type of negative regulation.

Also, the presence of glucose deactivates or inhibits adenylate cyclase that converts ATP to AMP. Hence no cAMP-CAP complex can form and bind to the CAP binding site of the DNA. In the absence of this complex, RNA polymerase enzyme can no longer initiate transcription efficiently.

Case 2: When only lactose is available:

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In this case, there is no simple sugar glucose is available and only the disaccharide lactose is available. There is a need to synthesize the enzymes.

A small amount of lactose is converted to allolactose. This compound is an inducer that binds to the repressor protein synthesized by Lac I gene. Thing binding of allolactose to repressor protein changes the conformation of repressor protein in such a way that it can no longer bind to the operator site on the DNA, i.e. operator site remains empty.

If there is no glucose in this case adenylate cyclase enzyme is not deactivated and cyclic AMP is produced from ATP. The cAMP binds with the CAP to form the cAMP-CAP complex which can bind to the CAP binding site on the DNA. The binding of this complex allows RNA polymerase to effectively initiate transcription of structural genes.

This is an example of positive regulation.

Case 3: When both lactose and glucose are available:

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In this case, though lactose is available, more preferable and simple sugar glucose is also available. So, till glucose is not completely finished, there is no need to synthesize the enzymes.

Since adenylate cyclase is deactivated in the presence of glucose, no cyclic AMP will be produced and hence no cAMP-CAP complex can bind to the CAP binding site. Therefore RNA polymerase enzyme is unable to initiate transcription of structural genes efficiently.