User:Claire.Ath/sandbox

=Gal Operon=

The gal operon, aka the gal regulon or the Leloir pathway, is a collection of genes responsible for galactose metabolism which are controlled by a collective promoter. Galactose metabolism is used in many bacteria, like E. coli, for construction of the cell wall.

Structure
The operon consists of four genes: gal K (galactokinase), gal T (galactose-1-phosphate uridylyltransferase), gal E (uridine-diphosphogalactose-4-epimerase), and gal M (mutarotase). Expression of these genes is regulated by a promoter, further broken down into the promoters P1 and P2, and two operators Oe (external operator) and Oi (internal operator).

Regulation
Gal R (galactose repressor) regulates gal operon expression by the concentration of galactose and glucose in the cell. Repression also involves gal S, an isorepressor. Gal S binds to gal R and aids in repression of the operon. When the concentration of galactose is high, D-galactose binds allosterically to gal R, causing it to dissociate from the operators and induce expression. In low levels of galactose, gal R remains bound to the operator, inhibiting expression. Glucose is the preferred catabolite for many bacteria. The presence of glucose indirectly lowers the concentration of cAMP, which forms a complex with CRP. This cAMP-CRP complex represses expression of gal S and induces expression of the gal operon. This mechanism of catabolite repression is also present in both the lac and ara operons.