Arabinose Operon

Arabinose operon is the operon system found in the bacterial cell which is used to breakdown the arabinose. This kind of operon provides energy to the cell by the breakdown of arabinose into Xylulose phosphate.

Arabinose provides energy in the form of a carbon source to the bacterial cell. The bacteria utilize this carbon source to perform various functions. In the arabinose operon, there are certain types of genes which perform specific functions inside the cell.

The main purpose of arabinose operon is to break the complex arabinose molecule into Xylulose phosphate which then enters the metabolic pathway namely the pentose phosphate pathway.

Content: Arabinose Operon

  1. Definition of Arabinose Operon
  2. Structural Elements
  3. Regulation of Arabinose Operon
  4. Conclusion

Definition of Arabinose Operon

Arabinose operon can define as the system which carries the number of genes like a regulatory, promoter, operator, inducer and structural genes for the breakdown of L-Arabinose into Xylulose phosphate.

The conversion of arabinose into xylulose phosphate is carried out by the structural genes ara-B, A and D which transcribes mRNA by the kinase, isomerase and epimerase enzymes respectively.

The enzymes kinase, isomerase and epimerase catalyze the conversion of L-arabinose to Xylulose phosphate which then enters the pentose phosphate pathway.

Structural Elements

The structure of arabinose operon is linear, and consist of four specific genes with a catabolic active site.

structure of arabinose operon

  • Structural genes
  • Inducer genes
  • Catabolic active site
  • Operator genes
  • Promoter genes
  • Regulatory gene

Structural Genes

There are three types of structural genes namely ara-B, ara-A and ara-D. The main function of the structural gene is to encode the metabolic enzymes. The metabolic enzymes are Kinase, Isomerase and Epimerase which are synthesized by the ara-B, ara-A and ara-D respectively. These metabolic enzymes breakdown the non-glucose molecule i.e. Arabinose to produce a “Multigenic or Polycistronic mRNA”.

Inducer Gene

There is an inducer gene in arabinose operon namely ara I gene. The ara I gene the transcription. In arabinose operon, the inducer molecule is the Arabinose, which binds with the repressor protein to induce the transcription of the gene into mRNA. In the presence of inducer, the arabinose operon works positively but in the absence of inducer, it works negatively.

Catabolic Active Site

In arabinose operon, there is an activator site refer as “CAP”. CAP stands for “Catabolite activator protein” which activates the efficiency of transcription rate by promoting the effective binding of RNA polymerase to the promoter region. When the availability of glucose is high with low arabinose, then ATP will not convert into cAMP. When the availability of glucose is low with high arabinose content, then ATP will convert into cAMP which will bind to the CAP to activate the transcription of mRNA. The Cyclic AMP plus CAP will form a complex which binds to the CAP region of the operon.

Operator Gene

There are two operator gene namely O1 and O2. These genes operate araBAD mRNA synthesis. In positive regulation, when the amount of glucose is low and the amount of arabinose is high, the repressor protein (Ara C) will get activated by the arabinose and will promote the synthesis of araBAD mRNA. In negative regulation, when the amount of glucose is high and the amount of arabinose is low, the repressor protein (Ara C) will not promote the synthesis of araBAD mRNA.

Promoter Gene

There are two promoter genes in the arabinose operon namely PBAD and PC. PBAD is the promoter site of ara BAD structural genes which promotes the synthesis of ara BAD mRNA. PC is the promoter site of ara C regulatory gene which promotes the synthesis of Ara C repressor protein. The Ara C repressor exists in active P1 state in the absence of inducer while in presence of inducer exist in inactive P2 state.

Regulatory gene

The ara-C is the only regulatory gene refers in the arabinose operon. The ara-C gene encodes the Ara C protein which acts as a “Repressor”. The Ara C protein regulates the Arabinose operon both positively and negatively. When the Ara C protein binds with the operator, it will repress the synthesis of araBAD mRNA and will not promote the binding of RNA polymerase to the promoter region. When the Ara C repressor protein binds with the inducer i.e. Arabinose, then the complex becomes activated. This activation of Ara C with the Arabinose will promote the attachment of RNA polymerase to the promoter region and leads to the synthesis of araBAD mRNA.

Regulation of Arabinose Operon

The arabinose operon is regulated both positively and negatively like a Lac-operon model.

Positive Regulation

To make it clear and simple, here the POSITIVE term indicates there will be a synthesis of mRNA. Therefore, in positive regulation, the ara BAD mRNA will produce. Arabinose operon is positively regulated by two conditions which are given below:

Case-I when both inducer and repressor protein is absent:

positive regulation of arabinose operon case 1

When both inducer and repressor protein is absent, there will be no repression of the Arabinose operon. In this condition, the RNA polymerase will bind with the specific promoter region and transcribe the ara-BAD genes to form mRNA. But in this case, the rate of transcribing mRNA is much slower.

Case-II when both inducer and repressor are present:

positive regulation of arabinose operon case 2

The inducer (Arabinose) will bind with the repressor protein to regulate the mRNA transcription. The Ara-C protein by forming a complex with the inducer will not able to form a loop. The arabinose binds with the Ara-C dimer and changes its structural configuration. This change in structural configuration will allow the RNA polymerase to transcribe the ara-BAD genes to form mRNA. The mRNA will further translate into proteins.

Negative Regulation

Now here the term NEGATIVE term indicates that there will be no transcription. To understand the process in a simple way, let us take condition III to compare it with the before two conditions.

Case-III when only repressor protein is present:

Negative regulation of arabinose operon

The main role of arabinose operon is to breakdown the arabinose. And if there is no arabinose, there will be no transcription and translation of the DNA molecule. Arabinose acts as an “Inducer” which binds with the repressor protein ARA-C to inactivate it. But in the absence of inducer, the repressor protein will be produced by the ara-C gene. ARA-C repressor protein forms a dimer with the operator and the inducer gene by forming a loop. The loop formation will not allow the RNA polymerase to transcribe the ara-BAD genes to form mRNA.

Conclusion

Therefore the arabinose operon can be both positively and negatively regulated. In other words, we can say that this type of operon system can be controlled by both activation and repression. The repression of the operon is through the repressor protein i.e. Ara-C protein. The activation of the operon is through the inducer.

The Arabinose will bind with the repressor protein and inactivates it into the P2 state, which can bind with the inducer and the operator genes. So, we can conclude that the arabinose operon is switched off in the presence of repressor and switched on in the presence of inducer.

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