7. Structure of the lac operon
• The lac operon consists of three structural genes,
and a promoter, a terminator, regulator, and
an operator. The three structural genes
are: lacZ, lacY, and lacA.
• lacZ encodes β-galactosidase (LacZ), an
intracellular enzyme that cleaves
the disaccharide lactose into glucose and galactose.
8. • lacY encodes Beta-galactoside
permease (LacY), a
transmembrane symporter that pumps β-
galactosides including lactose into the cell using
a proton gradient in the same direction.
Permease increases the permeability of the cell
to β-galactosides.
• lacA encodes β-galactoside
transacetylase (LacA), an enzyme that transfers
an acetyl group from acetyl-CoA to β-
galactosides
14. • The promoter is the binding site for RNA polymerase, the
enzyme that performs transcription.
• The operator is a negative regulatory site bound by
the lac repressor protein. The operator overlaps with the
promoter, and when the lac repressor is bound, RNA
polymerase cannot bind to the promoter and start
transcription.
• The CAP binding site is a positive regulatory site that is
bound by catabolite activator protein (CAP). When CAP is
bound to this site, it promotes transcription by helping
RNA polymerase bind to the promoter.
15.
16. • The regulatory gene lac I produces an mRNA that
produces a Lac repressor protein, which can bind to
the operator of the lac operon.