lac operon

lacZ gene

Position: First gene in the operon Function: Encodes β-galactosidase, which breaks lactose into glucose and galactose

lacY gene

Position: After lacZ

Function: Encodes permease, which transports lactose into the cell

lacA gene

Position: After lacY

Function: Encodes transacetylase, involved in lactose metabolism (exact role unclear)

LacI (Repressor) – Negative Control

Role/Action: a regulatory gene that produces the Lac repressor protein.

CAP (Catabolite Activator Protein) – Positive Control

Role/Action: CAP enhances transcription of the lac operon when glucose levels are low.

LacI (Repressor) – Negative Control

Effect: This prevents RNA polymerase from transcribing the structural genes, effectively turning off the operon.

CAP (Catabolite Activator Protein) – Positive Control

Effect: This increases the affinity of RNA polymerase for the promoter, boosting transcription.

LacI + no lactose

operon OFF (negative control)

LacI + lactose

operon ON

CAP + low glucose

transcription enhanced (positive control)

CAP + high glucose

transcription reduced

lacI (Regulatory gene)

Position: Upstream of the lac operon Function: Encodes the lac repressor protein, which binds to the operator to inhibit transcription

Transcribed: Yes, but separately from the operon

Promoter (lacP)

Position: Just upstream of the operator and structural genes

Operator (lacO)

Position: Between the promoter and structural genes

Operator lac(O)

Function: Binding site for the lac repressor; blocks RNA polymerase when bound

Promoter (lacP)

Function: Binding site for RNA polymerase to initiate transcription

negative control

requires lactose (allolactose) for inactivation of repressor

positive control

requires cAMP (low glucose), activates CAP-cAMP complex