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The operon is a set of coding regions of DNA clustered together that includes structural genes and it is under the control of a single regulatory region. The operator regulates transcription, which is a repressor protein. When the operator binds to a segment of the regulatory region, transcription is shut down.
E. Coli will be used as an example of how an inducible operon works. E. Coli's main source of nutrition is glucose. If glucose is not available, it can utilize lactose. But the necessary enzymes used to digest lactose aren't normally made by E. Coli. When lactose is available in the environment, certain activities will take place allowing E. Coli to digest lactose. Lactose induces transcription of enzymes to utilize lactose.
It takes three enzymes for E. Coli to digest lactose: beta-glycosidase, permease, and transacetylase. The coding regions for these three enzymes are linked together under the control of a single regulatory region called the lac operon.
The I gene makes the lac repressor protein. The I gene constitutively transcribes mRNA for the repressor protein. The repressor is a regulator protein. It can bind to either lactose or the operator DNA. The binding of a repressor protein to either DNA or lactose is reversible, so the repressor is bound most often to whichever of the two it finds in the highest concentration. The lac repressor protein is highly attracted to the operator sequence of the lac promoter. When it is bound, transcription does not occur. If lactose levels rise to high concentrations, the lactose molecules will quickly bind to a lactose molecule rather than the one operator. The operator would no longer be repressed and transcription will proceed.
The coding region of the lac operon is transcribed as a single polycistronic mRNA. When the three lactose-digesting enzyme are available, they will digest the lactose. Lactose levels will go down and the repressor molecule will bind to the operator when there is no more lactose to metabolize. Also as the availability of lactose gets low, the lactose molecules that were bound to repressor molecules can be degraded. This frees up repressor molecules. When the enzymes are no longer needed, transcription is shut down.
Lactose alone cannot induce the lac operon. E. Coli prefers to utilize glucose rather than lactose because glucose is simpler and more abundant.
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"How the Operon Works." 123HelpMe.com. 11 Nov 2019
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The lac promoter by itself binds poorly to RNA polymerase. So even if the repressor is not bound to the operator, transcription does not readily occur. Therefore the presence of lactose alone does not induce transcription. When catabolite activator protein of CAP, an accessory protein binds to the promoter it stimulates the interaction of RNA polymerase and transcription. CAP binds to the promoter only when there is no glucose in the environment. When glucose levels are low, there is an increase in the level of cyclic AMP or camp, which is metabolic derivative of ATP. cAMP binds to CAP and then Cap binds to the lac promoter and transcription is stimulated. The lac operon will be transcribed only when lactose is present and there in no glucose.
With a repressible operon we need a situation where the products of an operon are normally needed, and only under unusual circumstances would the operon need to be shut down. E coli normally has to make its own tryptophan. Tryptophan is rarely found in the environment. The biosyntheticf pathway for the production of tryptophan consists of five enzymatic steps. The coding regions for each other the enzymes in the pathway are located within the operon. The promoter of the trp operon is highly attracted to RNA polymerase. No additional protein is needed to transcribe the operon. The operon is constitutively transcribed. If tryptophan is present in the environment, the bacterium will inactivate the operon and utilize the free tryptophan.
The trp repressor is incapable of binding to the trp operator itself. But when there is free trptophan around, the tryptophan acts as a co-repressor. It binds to the trp repressor and forms a repressor complex. This complex binds to the operator and shuts down transcription.
If tryptophan levels decline, the repressor is less likely to be found to a tryptophan molecule. The operator is freed and transcription continues again.
Bacteria have complex methods to regulator the expression of certain genes. The operon shows that even though bacteria may seem like simple organism, they have evolved complex systems that have allowed them to survive for billions of years.