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 particlular the cell wall component lipopolysa