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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 (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.

Structural Genes
Bacterial operons are polycistronic, meaning multiple gene products can be translated from one mRNA transcript. The gal operon possesses four structural genes which appear in the order: galE, galT, galK, and galM. As a result, whenever galactose metabolism is needed, the gal operon will be transcribed and the four gene products will then be translated. The four 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.
 * 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 gene products, however, do not function in the order in which they appear. 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 four structural genes in the gal operon function in reverse order to their orientation on the operon.

Regulatory Structures
Upstream of the three aforementioned structural genes, the gal operon has two overlapping promoters, which appear in the order: P2 and P1. Both of the promoters have an associated transcription initiation site that appear five base pairs apart and are different in sequence. .

As well, the gal operon is associated with two operators, which appear in the order: OE and OI where E represents external and I represents internal. OI, the internal operator, is a structure that occurs within the structural gene galE. 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.



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 (crp) that when bound together with cAMP, forms a cAMP-CRP complex which stimulates transcription at P1 and represses transcription at P2

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 gal operon is approximately proportionate between the sites. However, when the cAMP-crp complex is present, transcription initiation predominantly begins at P1, making up more than >95% of transcription intiations. 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.

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. This repressosome is stabilized by the binding of an HU protein to the HBS (HU binding site) located between the promoters and OI.



The galR protein, however, if binding to OE alone, stimulates transcription initiation from P2 and partially represses transcription initiation from P1.

The galS Protein
The galS protein is a member of the GalR-LacI family, the same family in which the GalR protein belongs. GalS is 53% identical and 85% similar with GalR. As a result, GalS represses transcription of the operon in an analagous manner to GalR, forming the repressosome and inducing HU protein binding. However, it represses transcription much less efficiently in comparison to GalR.