Lactose Operon Regulation

The Lactose Operon

• The lactose operon is a set of genes in bacteria that are regulated by the repressor protein.
• The operon consists of three structural genes: lacZ, lacY, and lacA.
• The genes are controlled by their own promoter (Plac) and an operator sequence.

Regulation of the Lactose Operon

• The lac repressor protein binds to the operator sequence, preventing transcription of the structural genes.
• There are three possible binding sites for the repressor: O1, O2, and O3.
• The repressor binds to O1 and either of the other two sites.

Induction of the Lactose Operon

• The inducer is allolactose, a minor reaction product of β-galactosidase.
• Allolactose binds to the repressor, causing it to be released from the operator, and transcription of the structural genes is turned on.
• Binding of the inducer to the repressor is allosteric, meaning it binds to a site other than the active site.

Lactose Metabolism in E. coli

• Lactose metabolism requires the activities of galactoside permease (uptake) and β-galactosidase (metabolism).
• β-galactosidase also catalyses the conversion of lactose to allolactose by transglycosylation.
• When glucose is abundant and lactose is lacking, cells produce low levels of enzymes for lactose metabolism.
• When lactose is present and glucose is absent, cells can use lactose as their energy source.

Induction of Genes for Lactose Utilization

• Gene expression is induced when glucose is absent and lactose is present.
• The graph shows the activity of enzymes involved in lactose utilization over time as lactose is added.

The Lactose Operon Genes

• lacZ: encodes β-galactosidase.
• lacY: encodes galactoside permease.
• lacA: encodes thiogalactoside transacetylase.