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Pathway Description
Lac Operon
Escherichia coli
Category:
Metabolite Pathway
Sub-Category:
Signaling
Created: 2015-06-26
Last Updated: 2019-08-16
The lac operon in E. coli produces three proteins that are used to metabolize lactose in the absence of glucose. If glucose is present in the cell, cAMP levels will be low, and only a small amount will be able to bind to the cAMP-activated global transcriptional regulator (CRP or CAP). Without cAMP bound, the protein is unactivated, and cannot bind to the activator binding site of the operon. However, when glucose levels are low, cAMP levels are higher, and more can bind to and activate CRP, allowing it to activate the operon. At the same time, if lactose levels in the cell are low, there will be minimal amounts of allolactose produced by any beta-galactosidase present currently in the cell. Allolactose is necessary to bind to the lactose operon repressor, and without allolactose bound to the repressor, it is tightly bound to the operator region of the operon. However, when concentrations of lactose are higher in the cell, more allolactose is produced, and when it binds to the repressor, the repressor cannot bind to the operator, freeing it and allowing RNA polymerase to bind. This, combined with the binding of the CRP protein to the activator binding site leads to all three genes in the operon being transcribed.
The first gene, lacZ, encodes the protein beta-galactosidase, an enzyme that hydrolyzes beta-galactosides into monosaccharides. In this instance, it hydrolyzes lactose into glucose and galactose. In some cases, it can also cause the isomerization of lactose into allolactose.
The second gene, lacY, encodes the protein lactose permease, which is a transport protein that pumps lactose into the cell by using a proton gradient that also flows into the cell.
Finally, the last gene, lacA, encodes the protein galactoside O-acetyltransferase, an enzyme that catalyzes the transfer of the acetyl group of acetyl-CoA to beta-galactosides. This specific protein is not known to be important to the lac operon, but may be important in detoxifying the bacteria by preventing acetylated galactosides from re-entering the cell.
References
Lac Operon References
Marbach A, Bettenbrock K: lac operon induction in Escherichia coli: Systematic comparison of IPTG and TMG induction and influence of the transacetylase LacA. J Biotechnol. 2012 Jan;157(1):82-8. doi: 10.1016/j.jbiotec.2011.10.009. Epub 2011 Nov 3.
Pubmed: 22079752
Hediger MA, Johnson DF, Nierlich DP, Zabin I: DNA sequence of the lactose operon: the lacA gene and the transcriptional termination region. Proc Natl Acad Sci U S A. 1985 Oct;82(19):6414-8. doi: 10.1073/pnas.82.19.6414.
Pubmed: 3901000
Misumi Y, Ogata S, Ohkubo K, Hirose S, Ikehara Y: Primary structure of human placental 5'-nucleotidase and identification of the glycolipid anchor in the mature form. Eur J Biochem. 1990 Aug 17;191(3):563-9. doi: 10.1111/j.1432-1033.1990.tb19158.x.
Pubmed: 2129526
Hansen KR, Resta R, Webb CF, Thompson LF: Isolation and characterization of the promoter of the human 5'-nucleotidase (CD73)-encoding gene. Gene. 1995 Dec 29;167(1-2):307-12. doi: 10.1016/0378-1119(95)00574-9.
Pubmed: 8566797
Knapp K, Zebisch M, Pippel J, El-Tayeb A, Muller CE, Strater N: Crystal structure of the human ecto-5'-nucleotidase (CD73): insights into the regulation of purinergic signaling. Structure. 2012 Dec 5;20(12):2161-73. doi: 10.1016/j.str.2012.10.001. Epub 2012 Nov 8.
Pubmed: 23142347
Kalnins A, Otto K, Ruther U, Muller-Hill B: Sequence of the lacZ gene of Escherichia coli. EMBO J. 1983;2(4):593-7.
Pubmed: 6313347
Sutendra G, Wong S, Fraser ME, Huber RE: Beta-galactosidase (Escherichia coli) has a second catalytically important Mg2+ site. Biochem Biophys Res Commun. 2007 Jan 12;352(2):566-70. doi: 10.1016/j.bbrc.2006.11.061. Epub 2006 Nov 20.
Pubmed: 17126292
Blattner FR, Plunkett G 3rd, Bloch CA, Perna NT, Burland V, Riley M, Collado-Vides J, Glasner JD, Rode CK, Mayhew GF, Gregor J, Davis NW, Kirkpatrick HA, Goeden MA, Rose DJ, Mau B, Shao Y: The complete genome sequence of Escherichia coli K-12. Science. 1997 Sep 5;277(5331):1453-62. doi: 10.1126/science.277.5331.1453.
Pubmed: 9278503
Ujwal ML, Jung H, Bibi E, Manoil C, Altenbach C, Hubbell WL, Kaback HR: Membrane topology of helices VII and XI in the lactose permease of Escherichia coli studied by lacY-phoA fusion analysis and site-directed spectroscopy. Biochemistry. 1995 Nov 14;34(45):14909-17. doi: 10.1021/bi00045a036.
Pubmed: 7578103
Buchel DE, Gronenborn B, Muller-Hill B: Sequence of the lactose permease gene. Nature. 1980 Feb 7;283(5747):541-5. doi: 10.1038/283541a0.
Pubmed: 6444453
Fowler AV, Hediger MA, Musso RE, Zabin I: The amino acid sequence of thiogalactoside transacetylase of Escherichia coli. Biochimie. 1985 Jan;67(1):101-8. doi: 10.1016/s0300-9084(85)80235-2.
Pubmed: 3922433
Farabaugh PJ: Sequence of the lacI gene. Nature. 1978 Aug 24;274(5673):765-9. doi: 10.1038/274765a0.
Pubmed: 355891
Hayashi K, Morooka N, Yamamoto Y, Fujita K, Isono K, Choi S, Ohtsubo E, Baba T, Wanner BL, Mori H, Horiuchi T: Highly accurate genome sequences of Escherichia coli K-12 strains MG1655 and W3110. Mol Syst Biol. 2006;2:2006.0007. doi: 10.1038/msb4100049. Epub 2006 Feb 21.
Pubmed: 16738553
Cossart P, Gicquel-Sanzey B: Cloning and sequence of the crp gene of Escherichia coli K 12. Nucleic Acids Res. 1982 Feb 25;10(4):1363-78. doi: 10.1093/nar/10.4.1363.
Pubmed: 6280141
Aiba H, Fujimoto S, Ozaki N: Molecular cloning and nucleotide sequencing of the gene for E. coli cAMP receptor protein. Nucleic Acids Res. 1982 Feb 25;10(4):1345-61. doi: 10.1093/nar/10.4.1345.
Pubmed: 6280140
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