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==Regulation== |
==Regulation== |
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===cAMP-crp Complex=== |
===The cAMP-crp Complex=== |
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The cAMP-crp complex binds upstream of the two promoters and causes preferential transcription starting at P<sub>1</sub> rather than from both P<sub>1</sub> and P<sub>2</sub>. In the absence of the cAMP-crp complex, transcription initiation of the ''gal''operon is approximately proportionate between the sites. However, when cAMP-crp is present, transcription initiation predominantly begins at P<sub>1</sub> (>95% of transcription)<ref name="Weickert&Adhya) /><ref name="Goodrich&McClure">{{cite journal|last1=Goodrich|first1=James A.|last2=McClure|first2=William R.|title=Regulation of open complex formation at the Escherichia coli galactose operon promoters: Simultaneous interaction of RNA polymerase, gal repressor and CAP/cAMP|journal=Journal of Molecular Biology|date=1992|volume=224|issue=1|page=15-29|doi=10.1016/0022-2836(92)90573-3|url=http://www.sciencedirect.com/science/article/pii/0022283692905733}}</ref>. |
The cAMP-crp complex binds upstream of the two promoters and causes preferential transcription starting at P<sub>1</sub> rather than from both P<sub>1</sub> and P<sub>2</sub>. In the absence of the cAMP-crp complex, transcription initiation of the ''gal''operon is approximately proportionate between the sites. However, when the cAMP-crp complex is present, transcription initiation predominantly begins at P<sub>1</sub> (>95% of transcription intiations)<ref name="Weickert&Adhya) /><ref name="Goodrich&McClure">{{cite journal|last1=Goodrich|first1=James A.|last2=McClure|first2=William R.|title=Regulation of open complex formation at the Escherichia coli galactose operon promoters: Simultaneous interaction of RNA polymerase, gal repressor and CAP/cAMP|journal=Journal of Molecular Biology|date=1992|volume=224|issue=1|page=15-29|doi=10.1016/0022-2836(92)90573-3|url=http://www.sciencedirect.com/science/article/pii/0022283692905733}}</ref>. |
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The cAMP-crp complex alters transcription initiation preference by (1) increasing RNA polymerase affinity to P<sub>1</sub>, thereby increasing RNA polymerase binding to P<sub>1</sub>, and (2) elevating the rate of conversion of the closed DNA complex to an open DNA complex that is stable and ready to be transcribed<ref name=Weickert&Adhya" /><ref name="Goodrich&McClure" />. |
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===The galR Protein=== |
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The galR protein works to repress both P<sub>1</sub> and P<sub>2</sub> by forming a repressosome. GalR proteins come together to form [[dimer|dimers]], of which each monomer component binds simultaneously to either O<sub>E</sub> or O<sub>I</sub>. This interaction of dimers to operators is required for full repression and alters DNA conformation by forming a loop- the repressosome. <ref name="Semsey&Virnik&Adhya">{{cite journal|last1=Semsey|first1=Szabolcs|last2=Virnik|first2=Konstantin|last3=Adhya|first3=Sankar|title=Three-stage Regulation of the Amphibolic gal Operon: From Repressosome to GalR-free DNA|journal=Journal of Molecular Biology|date=2006|volume=358|page=355-363|doi=10.1016/j.jmb.2006.02.022|url=http://ac.els-cdn.com.proxy1.lib.uwo.ca/S0022283606002063/1-s2.0-S0022283606002063-main.pdf?_tid=ffb22c04-9a87-11e5-bf33-00000aacb35f&acdnat=1449234640_fc6443b90dd78809b5848daacbdc094f}}</ref> |
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The galR protein, however, if binding to O<sub>E</sub> alone, stimulates transcription initiation from P<sub>2</sub> and partially represses transcription initiation from P<sub>1</sub><ref name="Semsey&Virnik&Adhya" />. |
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==References== |
==References== |
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Revision as of 14:41, 4 December 2015
Remember to keep to an encyclopedic style that is accessible. Good references but try to find something more recent too. BenjaminLaufer (talk) 15:49, 4 November 2015 (UTC)
The gal operon (galactose operon) is a prokaryotic operon necessary for galactose transport and metabolism in Escherichia coli, and many other bacteria including those within the Streptomyces genus. Although galactose is not the preferred carbon source in bacteria, D-galactose (one of two possible isomers) is important in E. coli as a building block for other cellular pathways[1] (ie. lactose synthesis, glucose conversion via the Leloir pathway, etc.). The gal operon contains genes coding for the enzymes necessary in this galactose to glucose conversion as well as the controls necessary for this process[2].
 | This is a user sandbox of Rchan255. A user sandbox is a subpage of the user's user page. It serves as a testing spot and page development space for the user and is not an encyclopedia article. |
Gal Operon Structure
Structural Genes
Bacterial operons are polycistronic, meaning multiple gene products can be translated from one mRNA transcript[1]. The gal operon possesses four structural genes which appear in the order: galE, galT, galk, and galM[3][1]. As a result, whenever galactose metabolism is needed, the gal operon will be transcribed and the three gene products will then be translated. The three genes and their products are as follows:
- galE is the first gene to appear in the gal operon and codes for the enzyme UDP-Galactose-4-Epimerase, or GALE, which assists the conversion of UDP-galactose to UDP-glucose
- galT is the second gene to appear in the gal operon and codes for the enzyme Galactose-1-Phosphat-Uridyl-Transferase, or GALT, which assists the conversion of Galactose-1-P to UDP-Galactose
- galK is the third gene to appear in the gal operon and codes for the enzyme Galactokinase, which phosphorylates Galactose to Galactose-1-P[2].
- galM is the final gene to appear the gal operon and codes for the enzyme Mutarotase, which interconverts D-α-galactose and D-β-galactose.
The order of the genes, however, do not align with the order in which the respective gene products function in the galactose metabolic pathway. Instead, the order in which the gene products function is Mutarotase (galM), Galactokinase (galK), GALT (galT), then finally GALE (galE). Thus the gene products of the three structural genes in the gal operon function in reverse order in which they appear.
Regulatory Structures
Upstream of the three aforementioned structural genes, the gal operon has two overlapping promoters, which appear in the order: P2 and P1[3]. Both of the promoters have an associated transcription initiation site that appear five base pairs apart and are different in sequence.[4].
As well, the gal operon is associated with two operators, which appear in the order: OE and OI. These two operators are separated by 113 base pairs, straddling both the promoters and transcription initiation sites with OE appearing at the beginning of the operon, and OI appearing a few base pairs upstream of the first structural gene galE[4].
Regulatory Genes
Regulatory genes code for gene products that alter the expression of other genes, usually either inhibiting or activating expression. There are three regulatory genes that are not part of the gal operon but associate with it in a regulatory fashion:
- galR codes for the gal repressor, which regulates both P1 and P2
- galS codes for the gal isorepressor which is similar to the gal repressor and also regulates both P1 and P2
- crp codes for cyclic AMP receptor protein that when bound together with cAMP, forms a cAMP-CRP complex which stimulates transcription at P1 and represses transcription at P2[3]
Regulation
The cAMP-crp Complex
The cAMP-crp complex binds upstream of the two promoters and causes preferential transcription starting at P1 rather than from both P1 and P2. In the absence of the cAMP-crp complex, transcription initiation of the galoperon is approximately proportionate between the sites. However, when the cAMP-crp complex is present, transcription initiation predominantly begins at P1 (>95% of transcription intiations)[5][6]. The cAMP-crp complex alters transcription initiation preference by (1) increasing RNA polymerase affinity to P1, thereby increasing RNA polymerase binding to P1, and (2) elevating the rate of conversion of the closed DNA complex to an open DNA complex that is stable and ready to be transcribed[7][6].
The galR Protein
The galR protein works to repress both P1 and P2 by forming a repressosome. GalR proteins come together to form dimers, of which each monomer component binds simultaneously to either OE or OI. This interaction of dimers to operators is required for full repression and alters DNA conformation by forming a loop- the repressosome. [8] The galR protein, however, if binding to OE alone, stimulates transcription initiation from P2 and partially represses transcription initiation from P1[8].
References
1. The galactose regulon of Escherichia coli- [1]
2. Regulation of open complex formation at the Escherichia coli galactose operon promoters: Simultaneous interaction of RNA polymerase, gal repressor and CAP/cAMP- [6]
3. Regulation of the gal Operon of Escherichia coli by the capR Gene*- [9]
4. A control element within a structural gene: The gal operon of Escherichia coli- [3]
5. Control of transcription termination- [10]
6. Regulatory sequences involved in the promotion and termination of RNA transcription- [11]
7. Two promoters, one inducible and one constitutive, control transcription of the Streptomyces lividans galactose operon- [12]
8. Galactose Operon- [2]
9. Quantification of the galactose-operon mRNAs 5 bases different in their 5’-ends[4]
- ^ a b c d Weickert, Michael J.; Adhya, Sankar (1993). "The galactose regulon of Escherichia coli". Molecular Microbiology. 10 (2): 245-251. doi:10.1111/j.1365-2958.1993.tb01950.x.
- ^ a b c {cite web|title=Galactose Operon|url=http://www.spektrum.de/lexikon/biologie/galactose-operon/26298%7Cwebsite=Spektrum der Wissenschaft|publisher=Spektrum Akademischer Verlag|accessdate=11 November 2015}}
- ^ a b c d Irani, Meher H.; Orosz, Laszlo; Adhya, Sankar (March 1983). "A control element within a structural gene: The gal operon of Escherichia coli". Cell. 32 (3): 783-788. doi:10.1016/0092-8674(83)90064-8.
- ^ a b c Ji, Sang Chun; Jeon, Heung Jin; Yun, Sang Hoon; Lee, Hee Jung; Lim, Heon M. (July 2010). "Quantification of the galactose-operon mRNAs 5 bases different in their 5'-ends" (PDF). BMB Reports. 43 (7): 474-479. PMID 20663408.
- ^ Cite error: The named reference
Weickert&Adhya)was invoked but never defined (see the help page). - ^ a b c Goodrich, James A.; McClure, William R. (1992). "Regulation of open complex formation at the Escherichia coli galactose operon promoters: Simultaneous interaction of RNA polymerase, gal repressor and CAP/cAMP". Journal of Molecular Biology. 224 (1): 15-29. doi:10.1016/0022-2836(92)90573-3.
- ^ Cite error: The named reference
Weickert&Adhya"was invoked but never defined (see the help page). - ^ a b Semsey, Szabolcs; Virnik, Konstantin; Adhya, Sankar (2006). "Three-stage Regulation of the Amphibolic gal Operon: From Repressosome to GalR-free DNA" (PDF). Journal of Molecular Biology. 358: 355-363. doi:10.1016/j.jmb.2006.02.022.
- ^ Mackie, George; Wilson, David B. (May 25, 1972). "Regulation of the gal Operon of Escherichia coli by the capR Gene*". The Journal of Biological Chemistry. 247 (10): 2973-2978.
- ^ Adhya, Sankar; Gottesman, Max (1978). "Control of transcription termination". Annual Review of Biochemistry. 47: 967-996. doi:10.1146/annurev.bi.47.070178.004535.
- ^ Rosenberg, Martin; Court, Donald (1979). "Regulatory sequences involved in the promotion and termination of RNA transcription". Annual Review of Biochemistry. 13: 319-353. doi:10.1146/annurev.ge.13.120179.001535.
- ^ Fornwald, James A.; Schmidt, Francis J.; Adams, Craig W.; Rosenberg, Martin; Brawner, Mary E. (April 1987). "Two promoters, one inducible and one constitutive, control transcription of the Streptomyces lividans galactose operon" (PDF). Proceedings of the National Academy of Sciences. 84: 2130-2134. PMID 3031664.
