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Full Version: Regulation of Gene Expression : LAC Operon
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In 1961, Francis Jacob and Jacques Monad described Operon model for genetic control of lactose metabolism in E. coli. Operon refers to a group of genes that functions together to achieve a common task. Many bacterial genes function as operons. Vir operon in Ti plasmids is another example for an operon. Some operons are negatively induced while some are positively induced.

Lactose is one of the major carbohydrates and found in milk, known as milk sugar. It is a disaccharide consisting of Glucose and Galactose. Lactose does not easily diffuse across the E. coli cell membrane and must be actively transported into the cell by the enzyme permease. To utilize lactose as an energy source, E. coli must first break it into Glucose and Galactose, a reaction catalyzed by the enzyme β-galactosidase. This enzyme can also convert lactose into allolactose, a compound that plays an important role in regulating lactose metabolism. A third enzyme, thiogalactoside transacetylase, also is produced by lac operon, but its function in lactose metabolism is not yet known.

The lac operon is an example of a negative inducible operon. The enzymes β-galactosidase,permease and transacetylase are encoded by the structural genes in lac operon in E. coli. β-galactosidase is encoded by Lac Z gene, permease by Lac Y gene and transacetylase by Lac A gene. When lactose and glucose is absent in the medium, the rate of synthesis of all three enzymes simultaneously increases about a thousand within two or three minutes which is stimulated by a specific molecule, called as an inducer.

Although lactose appear to be the inducer here, allolactose is actually responsible for the induction. Lac Z gene, Lac Y gene and Lac A gene have a common promoter and are transcribed together. Upstream of the promoter is the regulator gene, Lac I, which has its own promoter. Lac I gene encodes a repressor protein. Each repressor protein consists of four identical polypeptides and has two binding sites; one site for binding with allolactose and the other for binding with DNA. In the absence of lactose, the repressor binds to the Lac operator site/Lac O and prevents the transcription of the lac genes by blocking binding of RNA polymerase.
When lactose is present, some of the lactose is converted into allolactose, which binds to the repressor and cause the repressor to be released from the DNA. Then the repressor is inactivated in the presence of lactose and the binding of RNA polymerase is no longer blocked. The transcription of Lac Z, Lac Y and Lac A takes place and the Lac enzymes are produced.

Repression never completely shuts down transcription of Lac operon. Even with the active repressor bound to the operator, there is low level of transcription and a few molecules of β-galactosidase, permease and transacetylase are synthesized.

When lactose appears in the medium; the permease present, transport a small amount of lactose into cell. There, the few molecules of β-galactosidase that are present, convert some of the lactose into allolactose. The allolactose then attaches to the repressor and alters its shape so that the repressor no longer binds to the operator. When the operator site is clear, RNA polymerase can bind and transcribe the structural genes of the operon.

Several compounds related to allolactose also can bind to the lac repressor and induce transcription of the Lac operon. One such inducer is isopropylthiogalactoside (IPTG). Although IPTG inactivated the repressor and allows the transcription of Lac Z, Lac Y and Lac A; this inducer is not metabolized by β-galactosidase.

The regulation of Lac operon is used in screening of competent cells Blue white selection. There, the mutants lack the ability to produce β-galactosidase whereas the non- transformed cells can produce β-galactosidase. The produced β-galactosidase will form a complex with X gal in the medium which appear in blue colour. IPTG acts as the inducer for the activation of lac genes.