Lac Operon-
Jacob and Monod (1961) gave the concept of operon model to explain the gene regulation in E.coli. Experiments on metabolization of glucose and lactose by E.coli were utilized in their study. The Lac operon is found to be present in E. coli and many other enteric bacteria. It is required for metabolism of lactose.
It is a well known fact that glucose is the most preferred carbon source utilized by living organisms including bacteria. If a bacteria is cultured on a medium in which both glucose and lactose are present, bacteria will first of all metabolize (utilize) glucose as a carbon source.
At the point when convergence of glucose will begin diminishing (or when glucose will be totally used by microscopic organisms) then, at that point, microbes will use lactose as a wellspring of energy. Presently to use lactose, microscopic organisms will require explicit compound called as β galactosidase.
Lac operon is the operon whose structural genes when expressed, it leads to production of enzyme β-galactosidase. Now it should be clear that β-galactosidase will be synthesized (from Lac operon) when lactose is present and glucose is absent.
Organization of lac operon
Structure of the lac operon-
The DNA of the lac operon contains (in order from left to right): CAP binding site, promoter (RNA polymerase binding site), operator (which overlaps with promoter), structural genes. There are three structural genes clustered together in Lac Operon, i.e. lacZ gene, lacY gene, and lacA gene.
The activator protein CAP (catabolite activator protein), when bound to a molecule called cAMP, at the CAP binding site, then it promotes RNA polymerase binding to the promoter. The lac repressor protein binds to the operator and blocks RNA polymerase from binding to the promoter and transcribing the operon.
Lac operon contain a regulator gene known as „lac I‟ which is present adjacent to structural gene and are responsible for regulation of expression of structural gene. Regulator gene which is responsible for the synthesis of protein, called as repressor protein. The repressor protein binds to specific DNA sequence (operator).
At the point when repressor protein ties to administrator, the limiting of RNA polymerase is forestalled (we realize that RNA polymerase is expected for record of qualities). Thus, there is no record of primary qualities and union of βgalactosidase is repressed.Consequently, the administrative quality codes for the repressor of the lac operon.
There are three structural genes present in Lac-
- operon called as LacZ, LacY and LacA. The LacZ gene codes for β-galactosidase, which is primarily responsible for the hydrolysis of the disaccharide lactose into glucose and galactose LacY codes for enzyme lactose permease, it is a protein which is required for transport of lactose into the cell through the cell membrane.
- LacA, the third structural gene codes for Galactoside O-acetyltransferase. Lac operon is an example of negatively regulator operon model and is under control of negative regulation.
Action of enzyme galactosidase in lac operon
A. When Lactose is absent in the cell-
When a lactose is absent inside the cell, there is no need of expression of genes responsible for metabolism of lactose. Repressor protein is synthesized from regulator gene and repressor protein bind to operator. As a result, binding of RNA polymerase DNA is prevented and hence there is no transcription of genes responsible for lactose metabolism.
Action of repressor protein on lac operon in absence of lactose
B. When Lactose is present in the cell-
- When lactose is present in the cell there is need of transcription of genes responsible for lactose metabolism. In this case, when regulator gene produces repressor protein, lactose molecule binds to repressor protein. Now, since repressor protein is bound to lactose, it cannot bind to operator.
- Hence, promoter site is free to be occupied by RNA polymerase, transcription of structural genes occurs and lactose is metabolized.
Action of repressor protein on lac operon in presence of lactose
Effect of cyclic AMP on Lac operon-
- CAP isn't always active (able to bind DNA). Instead, it's regulated by a small molecule called cyclic AMP (cAMP). The cAMP has been reported to have a positive control on lac operon. The cAMP is a "hunger signal" made by E. coli when glucose levels are low.
- Catabolite activator protein (CAP; also known as cAMP Receptor Protein, CRP) is a receptor protein found in E.coli, which binds to cAMP and forms a CRP-cAMP complex. This complex binds to promoter and facilitates binding of RNA polymerase to promoter.
- The cAMP binds to CAP, changing its shape and making it able to bind DNA and promote transcription. Without cAMP, CAP cannot bind DNA and is inactive.
- Hence presence of cAMP helps in transcription of structural genes (Lac Z, Lac Y, Lac A) resulting in synthesis of enzymes required for lactose metabolism.
- CAP is only active when glucose levels are low (cAMP levels are high). Thus, the lac operon can only be transcribed at high levels when glucose is absent. This strategy ensures that bacteria only turn on the lac operon and start using lactose after they have used up the entire preferred energy source (glucose).
Action of cAMP on lac operon
Catabolite repression-
- We have already seen that lac operon is under negative control. Beside this, it is also known that presence of glucose inhibits expression of lac operon.
- We have studied that when lactose is present, structural genes are synthesized (lac operon is on) but in presence of glucose, lac operon remains switched off whether lactose is present or not.
- This is known as catabolite repression.Enzyme Induction is still considered a form of negative control because the effect of the regulatory molecule (the active repressor) is to decrease or down regulate the rate of transcription.
- Catabolite repression is a type of positive control of transcription, since a regulatory protein affects an increase (up regulation) in the rate of transcription of an operon.
- The cycle was found in E. coli and was initially alluded to as the ―glucoseimpact" since it was found that glucose stifled the amalgamation of specific inducible compounds,despite the fact that the inducer of the pathway was available in the climate.
- The impact of theglucose catabolite is applied on a significant cell constituent called cyclic adenosinemonophosphate (cAMP).
- When glucose is present in high concentrations, the cAMP concentration is low; as the glucose concentration decreases, the concentration of cAMP increases correspondingly.
- The high concentration of cAMP is necessary for activation of the lac operon. Mutants that cannot convert ATP into cAMP cannot be induced to produce β-galactosidase, because the concentration of cAMP is not great enough to activate the lac operon.
- Likewise, there are different freaks that truly do make cAMP yet can't enact the laccatalysts, in light of the fact that these freaks need one more protein, called catabolite activator protein(CAP), made by the crp quality. CAP frames a complex with cAMP, and this complex isready to tie to the CAP site of the operon.
- The DNA-bound CAP is then able to interact physically with RNA polymerase and essentially increase the affinity of RNA polymerase for the lac promoter. In this way, the catabolite repression system contributes to the selective activation of the lac operon.
Catabolite control of lac operon
- The operon is inducible by lactose to the maximal levels when cAMP and CAP form a complex. Under conditions of high glucose, a glucose breakdown product inhibits the enzyme adenylate cyclase, preventing the conversion of ATP into cAMP.
- Under conditions of low glucose, there is no breakdown product, and therefore adenylate cyclase is active and cAMP is formed. When cAMP is present, it acts as an allosteric effector, complexing with CAP. The cAMP-CAP complex acts as an activator of lac operon transcription by binding to a region within the lac promoter.
