The gal operon is a prokaryotic operon, which encodes enzymes necessary for galactose metabolism. The operon contains two operators, OE (for external) and OI. The former is just before the promoter, and the latter is just after the galE gene (the first gene in the operon).
Repression of gene expression works via binding of repressor molecules to the 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.
The gal operon of E. coli consists of 3 structural genes: galE (epimerase), galT (galactose transferase), and galK (galactokinase), which are transcribed from two overlapping promoters PG1 and PG2 upstream from galE. Regulation of the operon is complex since the GalE product, an epimerase that converts UDP-glucose into UDP-galactose, 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.
The gal operon is controlled by CRP-cAMP as for the lac operon. CRP-cAMP binds to -35 region promoting transcription from PG1 but inhibiting transcription from PG2. When cells are grown in glucose, basal level transcription occurs from PG2. 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. When GalR binds as a dimer to the -60 site only, promoter PG2 is activated, not repressed, allowing basal levels of GalE to be produced. In this state promoter PG1 is inactivated through interactions with the alpha subunit of RNA polymerase.