Gene regulation in prokaryotes/The lac operon

lac operon structure
In the absence of lactose, the lac repressor binds to the operator and prevents transcription of the operon.

In the presence of lactose, the repressor cannot bind to the operator and lac proteins are produced. The gene product of the lac operon digests lactose. It is only present if lactose is present.

lac mutants
Lac proteins are produced constitutively in lacI mutants and laO mutants

Lac mutants are distinguished by performing a complementation testComplementation_test by creating a heterozygous mut1/mut2. If both gene mutations are the same functional unit, no functional gene product will be made. The phenotype will remain mutant. If the genes are in different functional units, they will complement to give a wild type phenotype.

Inducer mutant
lacI+ is dominant to lacI-. lacZ and lacY are inducible in lacI+/lacI- heterozygous mutants (lacI acts in cis and trans)

lacZ and lac Y are still inducible in lacI+lacY-lacZ-/lacI-lacY+lacZ+ mutants. LacI can act in trans.

Operator mutation
lacO- mutations are cis-dominant. lacZ and lacY are constitutive.

I-d mutation
I-d mutations are dominant to I+. The repressor functions as a tetramer and the mixed wild type and I-d tetramers are unable to bind DNA.

I-s mutation
I-s (super-repressed) mutations make the lac operon uninducible. The mutant repressor binds DNA but does not respond to inducer.

Lactose/Glucose
In the presence of glucose, the lac operon cannot be induced by addition of lactose. lac operon is controlled so that cells are prevented from wasting energy metabolizing complex sugars when easily metabolized sugars are present.

cAMP-CAP
A positively acting protein called Catabolite Activator Protein (CAP) stimulates RNA polymerase binding to the promotor by binding just upstream of the RNA polymerase binding site of the promotor by binding upstream of the RNA polymerase binding site. The repressor binding site overlaps the mRNA start site.

CAP uses cAMP as a small effector molecule. cAMP accumulates in cells as they run out of glucose. At high cAMP concentrations, cAMP binds to CAP and enables CAP to bind to the lac promoter.

cAMP-CAP is also used for induction in many other sugar metabolizing operons such as arabinose (ara) and galactose (gal).