Gal operon

The gal operon is a prokaryotic operon, which encodes enzymes necessary for galactose metabolism.[1] Repression of gene expression for this operon works via binding of repressor molecules to two operators. These repressors dimerize, creating a loop in the DNA. The loop as well as hindrance from the external operator prevent RNA polymerase from binding to the promoter, and thus prevent transcription.[2] Additionally, since the metabolism of galactose in the cell is involved in both anabolic and catabolic pathways, a novel regulatory system using two promoters for differential repression has been identified and characterized within the context of the gal operon.

Structure

The gal operon of E. coli consists of 4 structural genes: galE (epimerase), galT (galactose transferase), galK (galactokinase), and galM (mutarotase) which are transcribed from two overlapping promoters, PG1 (+1) and PG2 (-5), upstream from galE.[3] GalE encodes for an epimerase that converts UDP-glucose into UDP-galactose. This is required for the formation of UDP-galactose for cell wall biosynthesis, in particular the cell wall component lipopolysaccharide, even when cells are not using galactose as a carbon/energy source.[4] GalT encodes for the protein galactosyltransferase which catalyzes the transfer of a galactose sugar to an acceptor, forming a glycosidic bond.[5] GalK encodes for a kinase that phosphorylates α-D-galactose to galactose 1-phosphate.[6] Lastly, galM catalyzes the conversion of β-D-galactose to α-D-galactose as the first step in galactose metabolism.[7]

The gal operon contains two operators, OE (for external) and OI (for internal). The former is just upstream of the promoter, and the latter is just after the galE gene (the first gene in the operon). These operators bind the repressor, GalR, which is encoded from outside the operator region. For this repressor protein to function properly, the operon also contains a histone binding site to facilitate this process.[8]

An additional site, known as the activating site, is found following the external operator, but upstream of PG2. This site serves as the binding region for the cAMP-CRP complex, which modulates the activity of the promoters and thus, gene expression.[9]

Mechanism

The unlinked galR gene encodes the repressor for this system. A tetrameric GalR repressor binds to 2 operators, one located at +55 and one located at -60 relative to the PG1 start site. Looping of the DNA blocks the access of RNA polymerase to promoters and/or inhibits formation of the open complex. This looping requires the presence of the histone-like protein, HU to facilitate the formation of the structure and allow for proper repression.[8] When GalR binds as a dimer to the -60 site only, the promoter PG2 is activated, not repressed, allowing basal levels of GalE to be produced. In this state, the PG1 promoter is inactivated through interactions with the alpha subunit of RNA polymerase.[2] Activity of this repressor protein is controlled based on the levels of D-galactose in the cell. Increased levels of this sugar inhibit the activity of the repressor by binding allosterically, resulting in a conformational change of the protein, which suppresses its interactions with RNA polymerase and DNA.[10] This induces the activity of the operon, which will increase the rate of galactose metabolism.

The gal operon is also controlled by CRP-cAMP, similarly to the lac operon. CRP-cAMP binds to the -35 region, promoting transcription from PG1 but inhibiting transcription from PG2. This is accomplished due to the location of the activation sequence. When CRP-cAMP binds the activating sequence, it blocks RNA polymerase from establishing an open complex with PG2, but enhances a closed complex with RNA polymerase at PG1. This represses the activity of the PG2 promoter, and increases the activity of the PG1 promoter.[9] When cells are grown in glucose, basal level transcription occurs from PG2.

See also

References
  1. Weickert, M. J.; Adhya, S. (October 1993). "The galactose regulon of Escherichia coli". Molecular Microbiology. 10 (2): 245–251. doi:10.1111/j.1365-2958.1993.tb01950.x. ISSN 0950-382X. PMID 7934815.
  2. Roy, Siddhartha; Semsey, Szabolcs; Liu, Mofang; Gussin, Gary N.; Adhya, Sankar (2004-11-26). "GalR represses galP1 by inhibiting the rate-determining open complex formation through RNA polymerase contact: a GalR negative control mutant". Journal of Molecular Biology. 344 (3): 609–618. doi:10.1016/j.jmb.2004.09.070. ISSN 0022-2836. PMID 15533432.
  3. Adhya, S.; Miller, W. (1979-06-07). "Modulation of the two promoters of the galactose operon of Escherichia coli". Nature. 279 (5713): 492–494. Bibcode:1979Natur.279..492A. doi:10.1038/279492a0. ISSN 0028-0836. PMID 221830. S2CID 4260055.
  4. Holden, Hazel M.; Rayment, Ivan; Thoden, James B. (2003-11-07). "Structure and function of enzymes of the Leloir pathway for galactose metabolism". The Journal of Biological Chemistry. 278 (45): 43885–43888. doi:10.1074/jbc.R300025200. ISSN 0021-9258. PMID 12923184.
  5. Williams, Gavin J.; Thorson, Jon S. (2009). Natural product glycosyltransferases: properties and applications. Advances in Enzymology and Related Subjects of Biochemistry. Advances in Enzymology - and Related Areas of Molecular Biology. 76. pp. 55–119. doi:10.1002/9780470392881.ch2. ISBN 9780470392881. ISSN 0065-258X. PMID 18990828.
  6. Frey, P. A. (March 1996). "The Leloir pathway: a mechanistic imperative for three enzymes to change the stereochemical configuration of a single carbon in galactose". FASEB Journal. 10 (4): 461–470. doi:10.1096/fasebj.10.4.8647345. ISSN 0892-6638. PMID 8647345.
  7. Thoden, James B.; Kim, Jungwook; Raushel, Frank M.; Holden, Hazel M. (May 2003). "The catalytic mechanism of galactose mutarotase". Protein Science. 12 (5): 1051–1059. doi:10.1110/ps.0243203. ISSN 0961-8368. PMC 2323875. PMID 12717027.
  8. Aki, T.; Choy, H. E.; Adhya, S. (February 1996). "Histone-like protein HU as a specific transcriptional regulator: co-factor role in repression of gal transcription by GAL repressor". Genes to Cells: Devoted to Molecular & Cellular Mechanisms. 1 (2): 179–188. doi:10.1046/j.1365-2443.1996.d01-236.x. ISSN 1356-9597. PMID 9140062.
  9. Musso, R. E.; Di Lauro, R.; Adhya, S.; de Crombrugghe, B. (November 1977). "Dual control for transcription of the galactose operon by cyclic AMP and its receptor protein at two interspersed promoters". Cell. 12 (3): 847–854. doi:10.1016/0092-8674(77)90283-5. ISSN 0092-8674. PMID 200371. S2CID 37550787.
  10. Lee, Sang Jun; Lewis, Dale E. A.; Adhya, Sankar (December 2008). "Induction of the Galactose Enzymes in Escherichia coli Is Independent of the C-1-Hydroxyl Optical Configuration of the Inducer d-Galactose". Journal of Bacteriology. 190 (24): 7932–7938. doi:10.1128/JB.01008-08. ISSN 0021-9193. PMC 2593240. PMID 18931131.
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