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Lac Operon Concept in Bacteria

  • In this Lac Operon concept in bacteria post we have briefly explained about Lac Operon, structure and functions of Lac Operon of bacteria.

  • Each cell in the human contains all the genetic material for the growth and development of a human. Some of these genes will be need to be expressed all the time. These are the genes that are involved in of vital biochemical processes such as respiration. Other genes are not expressed all the time, they are switched on an off at need.

Lac Operon Concept in Bacteria

  • An operon is a group of functionally linked genes under the direction of a single operator. Operons are made up of many genes linked by a promoter and an operator. Operons are found in prokaryotes (bacteria and archaea) but not eukaryotes.
  • Multiple operons can be controlled by the same regulatory protein in some cases; in these cases, the operons form regulators. François Jacob and Jacques Monod discovered operons as a form of gene expression control in 1961.

LAC operon

  • The lactose operon (Lac operon) is a collection of genes with a single promoter that aids in the transcription of genes for lactose transport and metabolism in E. coli and other intestinal bacteria.
  • Gene regulation in prokaryotes is carried out using the Lac Operon model, in which E. coli and many other bacteria’s protein-coding genes are housed in a single transcription unit known as an operon, which shares the same transcriptional regulation but is translated separately. Changes in gene expression in prokaryotes are influenced by physiological and environmental factors.

Structural genes

LAC Operon

Lac Operon Concept in Bacteria

  • LacZ, lacY, and lacA are structural genes in the lac operon that are transcribed as a single mRNA in a single promoter. These lacZ, lacY, and lacA genes encode enzyme galactosidase, galactosidase permease, and thiogalactosidase transacetylase, respectively, which help the cell use lactose.
  • Lactose is transported into the cell by galactosidase permease. Lactose is hydrolyzed by galactosidase, which turns it into glucose and galactose. Thiogalactosidase transacetylase gives galactosides, glucosides, and lactosides an acetyl group.
  • Lactose is broken into monosaccharide, which can be utilised in glycolysis, by the  lacZ enzyme. LacY also aids in the transport of lactose into the cell via a membrane embedded transporter.

Regulatory Genes

  • The lac operon also has a number of regulatory DNA regions where proteins bind and control the operon’s transcription. Transcription is started by binding RNA polymerase to a single operator.
  • There is an operator, which is a negative regulatory site bound by the lac repressor protein, between the promoter and the structural genes. The operator overlaps the promoter when the lac repressor protein is attached, blocking the RNA polymerase.
  • The positive regulatory site – CAP binding site is occupied by catabolite activator protein (CAP), which aids RNA polymerase in binding to the promoter for further processing.

Conditions

  • E. coli can use either glucose, which is a monosaccharide, or lactose, which is a disaccharide. However, lactose needs to be hydrolysed (digested) first. So the bacterium prefers to use glucose when it can.
  • Lactose Absence
  • Lactose presence
  • High Glucose presence
  • Low Glucose presence
LAC Operon

The Lac operon and its control elements

Lactose Absence

LAC Operon
  • When lactose is not present, the lac repressor binds tightly to the operator just in front of the lac operon, the Operator site and prevents transcription of RNA polymerase and lacZ, lacY and lacA genes where the RNA polymerase settles before it starts transcribing.

Lactose Presence

LAC Operon
  • Within the bacterial cell, a small amount of the sugar allolactose is generated. This attaches to a different active site on the repressor protein (allosteric site). The repressor protein changes shape as a result of this (a conformational change).
  • In the presence of lactose, the lac repressor loses its capacity to bind DNA and floats off the operator, inhibiting transcription in the lac operon. Allolactose is an inducer that activates the lac operon, which is normally inactive. It is also classified as an inducible operon because it is normally turned off.

High glucose presence

  • E. coli does not require lactose as a sugar source when glucose is present in the media, hence the lac operon does not need to be active. The presence of glucose decreases the generation of cAMP from ATP by inhibiting the adenylate cyclase enzyme.
  • As a result, cAMP is not created, and CAP is unable to bind to DNA in the absence of cAMP, resulting in poor transcription. When glucose levels are low, E. coli produces cAMP as a hunger signal.

Low glucose presence

  • When glucose levels are low, E. coli releases a large amount of cAMP, which binds to and activates the CRP (cAMP receptor protein), which then changes shape, attaches to DNA, and stimulates transcription.
  • When glucose is unavailable, cAMP binds to CRP, forming a CRP-cAMP complex that activates the lac operon, allowing the cell to use lactose as a carbon source. Lac operon genes lacZ, lacY, and lacA are highly transcribed in the absence of glucose and in the presence of lactose.

Regulation

  • The lac operon is an excellent example of negative control (negative regulation) of gene expression because a binding repressor stops structural genes from being transcribed. When a regulatory protein binds to DNA and raises the rate of transcription, this is known as positive control or regulation of gene expression.
  • The regulating protein in this scenario is known as an activator. A good example of an activator is CAP/CRP, which regulates the lac operon. As a result, both negative and positive control is applied to the lac operon.

Further Readings

Reference