Question

3. Diagram the level of Beta Galactosidase in a bacterial culture (minimal medium) in the absence of lactose, then after lactose is added and again after lactose is removed (draw over time and note times of changed conditions). Do the same now for the level of lac mRNA.

Answer 1

When cells are grown in the presence of lactose, a lactose metabolite called allolactose, made from lactose by the product of the lacZ gene, binds to the repressor, causing an allosteric shift. Thus altered, the repressor is unable to bind to the operator, allowing RNA Polymerase to transcribe the lac genes and thereby leading to higher levels of the encoded proteins.

The lactose repressor hinders the production of β-galactosidase in the absence of lactose. The lacI gene coding for the repressor lies nearby the lac operon and is always expressed (constitutive). If lactose is missing from the growth medium, the repressor binds very tightly to a short DNA sequence just downstream of the promoter near the beginning of lacZ called the lac operator. The repressor binding to the operator interferes with binding of RNA Polymerase to the promoter, and therefore mRNA encoding LacZ and LacY is only made at very low levels.