Gene Regulation: Induction of Lactose Utilization

Induction of Genes for Lactose Utilization

• The activity of enzymes involved in lactose utilization increases rapidly after 10 minutes when lactose is added.
• Gene expression is induced when glucose is absent and lactose is present.

The Operon Model

• Consists of regulatory sequences, genes transcribed as a unit, an activator binding site, promoter, and repressor binding site (operator).

The Lactose Operon

• Contains 3 linked structural genes: lacz, lacy, and lacA, which encode β-galactosidase, galactoside permease, and thiogalactoside transacetylase, respectively.
• The 3 structural genes are under the control of their own promoter Plac and an operator sequence.
• Control of structural genes is by negative regulation, which utilizes a repressor.

Induction of the lac Operon

• In the absence of inducer, the repressor binds to the lac operator and represses transcription of the structural genes.
• The inducer is allolactose, a minor reaction product of β-galactosidase.
• The inducer binds to the repressor, causing the latter to be released from the operator, and thus transcription of the structural genes is turned on.

Lactose Metabolism in E.coli

• Uptake and metabolism of lactose require the activities of galactoside permease (uptake) and β-galactosidase (metabolism).
• β-galactosidase also catalyses conversion of lactose to allolactose by transglycosylation.
• When glucose is aplenty and lactose is lacking, cells make very low levels of enzymes for lactose metabolism.
• If cells are fed lactose but not glucose, cells can use it as their energy source given that lactose is effectively entering the cell and being hydrolysed into monosaccharides.

Regulation of the lac Operon

• The lac operon is negatively regulated by the lac repressor protein.
• The lac repressor protein binds to the operator region of the operon and prevents transcription of the genes.
• The lac operon is also positively regulated by the catabolite activator protein (CAP).
• The CAP protein binds to the activator region of the operon and promotes transcription of the genes.
• The lac operon is regulated by both glucose and lactose.
• The lac operon is a repressible operon because it is regulated by a repressor protein.