User:Yiktingg1/sandbox

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The L-arabinose operon, also called the ara or araBAD operon, is an

xylulose-5-phosphate.Cite error: The opening <ref> tag is malformed or has a bad name (see the help page
).

The structural genes of the L-arabinose operon are transcribed from a common

regulatory gene araC and the catabolite activator protein (CAP)-cAMP complex.[4] The regulator protein AraC is sensitive to the level of arabinose and plays a dual role as both an activator in the presence of arabinose and a repressor in the absence of arabinose to regulates the expression of araBAD.[5] AraC protein not only controls the expression of araBAD, but also auto-regulates its own expression at high AraC levels.[6]

Structure

L-arabinose operon is composed of structural genes and regulatory regions including the operator region (araO1, araO2) and the initiator region (araI1, araI2).[7] The structural genes, araB, araA and araD, encode enzymes for L-arabinose catabolism. There is also a CAP binding site where CAP-cAMP binds to and facilitates catabolite repression. i.e. positive regulation of araBAD.[8]

Structure of L-arabinose operon of E.coli

The regulatory gene,araC, is located upstream of L-arabinose operon and is encoded for arabinose-responsive

regulatory protein AraC. Both araC and araBAD have a discrete promoter where RNA polymerase bound and initiate transcription.Cite error: The opening <ref> tag is malformed or has a bad name (see the help page). In which, araBAD and araC are transcribed in opposite direction from the araBAD promoter (PBAD) and araC promoter (PC) respectively.Cite error: The opening <ref> tag is malformed or has a bad name (see the help page
).

Function

  • araA encodes for
    L-ribulose
    .
  • araB encodes for
    ribulose-5-phosphate
    .
  • araD encodes for
    xylulose-5-phosphate
    .
Metabolic pathway of L-arabinose via the action of three enzymes, which are encoded by Ara operon gene.
Catabolism of arabinose in E. coli
Substrate Enzyme(s) Function Reversible Product
L-arabinose AraA Isomerase Yes L-ribulose
L-ribulose AraB Ribulokinase No L-ribulose-5-phosphate
L-ribulose-5-phosphate AraD
Epimerase
 Yes D-xylulose-5-phosphate

In which, both L-ribulose 5-phosphate and D-xylulose-5-phosphate are involved in the pentose phosphate pathway and produce reducing power. Cite error: The opening <ref> tag is malformed or has a bad name (see the help page).

Regulation

Structure of AraC monomer

The L-arabinose system is not only under the control of CAP-cAMP activator, but also positively or negatively regulated through binding of AraC protein. AraC functions as a

dimerization domain.[9] The dimerization domain is responsible for arabinose-binding.[10]
AraC undergoes
allosteric inducer arabinose. [11]

AraC can also negatively autoregulates its own expression when the concentration of AraC becomes too high. AraC synthesis is repressed through binding of dimeric AraC to the operator region (araO1).

Negative regulation of araBAD

Negative regulation of L-arabinose operon via AraC protein

When arabinose is absent, cells do not need PBAD product for breaking down arabinose. Therefore, dimeric AraC acts as a

suppressor by which one monomer binds to the operator of the araBAD gene (araO2), another monomer binds to a distant DNA half site known as araI1.[12] This leads to the formation of a DNA loop. [13] This orientation blocks RNA polymerase from binding to the araBAD promoter.[14] Therefore, transcription of structural gene araBAD is inhibited.[15]

Positive regulation of araBAD

Positive regulation of L-arabinose operon via dimeric AraC and CAP

Expression of the araBAD operon is activated in the absence of glucose and in the presence of arabinose. When arabinose is present, both AraC and CAP work together and function as activators.[16]

Via AraC

AraC acts as an

allosterically induced by arabinose. One of the AraC monomers places near to the araBAD promoter in this configuration, which helps to recruit RNA polymerase to the promoter to initiate transcription.[17]

Via CAP/cAMP (Catabolite Repression)

CAP act as a transcriptional activator only in the absence of preferred sugar, glucose. [18] When glucose is absent, high level of CAP protein/cAMP complex bind to CAP binding site, a site between araI1 and araO1.[19] Binding of CAP/cAMP is responsible for opening up the DNA loop between araI1 and araO2, also increase the binding affinity of AraC protein for araI2. Thereby, promoting RNA polymerase to bind to araBAD promoter and switches on expression of araBAD required for metabolising L-arabinose.

Autoregulation of araC expression

Autoregulation of AraC

The expression of araC is negatively regulated by its own protein product, AraC. The excess AraC binds to the operator of the araC gene, araO1, at high AraC levels, which physically blocks the RNA polymerase from accessing the araC promoter. [20] Therefore, prevents the transcription of araC from araC promoter. i.e. AraC protein inhibits its own expression at high concentrations. Cite error: The opening <ref> tag is malformed or has a bad name (see the help page).

Use in protein expression system

L-arabinose operon has been a focus for research in

expression system, as the araBAD promoter can be used for producing an excessive level of targeted expression under tight regulation. By fusing araBAD promoter to a gene of interest, the expression of the target gene can be solely regulated by arabinose. i.e. constitutive expression of the gene of interest is permitted as long as arabinose is present. Cite error: The opening <ref> tag is malformed or has a bad name (see the help page
).

See also

Other Operon system

Reference

  1. ISBN 978-0470-57095-1. {{cite book}}: |edition= has extra text (help)CS1 maint: multiple names: authors list (link
    )
  2. doi:10.1016/S0168-9525(00)02153-3
    .
  3. ISBN 9780321507815. {{cite book}}: |edition= has extra text (help
    )
  4. doi:10.1111/j.1574-6976.2010.00226.x
    .
  5. PMID 2237403
    .
  6. doi:10.1002/bies.10237
    .
  7. doi:10.1016/0022-2836(75)90200-4
    .
  8. PMID 6251457
    .
  9. PMID 8516313
    .
  10. PMID 9600837
    .
  11. ISBN 9781429276344. {{cite book}}: |edition= has extra text (help
    )
  12. doi:10.1016/0022-2836(76)90120-0
    .
  13. doi:10.1006/jmbi.1998.1713
    .
  14. doi:10.1016/0022-2836(84)90241-9
    .
  15. ISBN 9780071316866. {{cite book}}: |edition= has extra text (help
    )
  16. ISBN 9781555816278. {{cite book}}: |edition= has extra text (help
    )
  17. ISBN 9780071102155.{{cite book}}: CS1 maint: extra punctuation (link
    )
  18. ISBN 9781464102257. {{cite book}}: |edition= has extra text (help
    )
  19. ISBN 0716743825. {{cite book}}: |edition= has extra text (help
    )
  20. PMID 6262769
    .

External links
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