Question

Predict the level of gene expression of the lac operon, as well as the status of the lac repressor and the CAP protein, when bacterial growth media contains the following sugars: a) no lactose or glucose, b) lactose but no glucose, c) glucose but no lactose, d) both lactose and glucose.

Answer 1

A. When there is no lactose or glucose in the medium, then there will be no source of energy for the bacteria and therefore it will die.

B. If there is lactose in the medium but no glucose, then Lac Operon will express itself. Lactose which is the inducer of Lac Operon will bind to lac repressor and degrade it. This will make operator free and now RNA polymerase will be able to transcribe the Operon. In the absence of glucose, CAP forms complexes with cAMP, and help Lac Operon to express itself.

C. If the lactose is not in the medium, then there is no inducer molecule to bind to and degrade Lac repressor. In this case, repressor will remain associated with operator and RNA polymerase will not be able to transcribe the Operon. The bacteria will use glucose as the source of energy. The levels of CAP will be very low and there will be no formation of CAP cAMP complexes and therefore no expression of the Operon.

D. If both lactose and glucose are present in the medium, then the preferred source of energy will be glucose. It is because the bacteria will not waste is energy in transcribing the Operon. The Operon will be switched off and repressor will be bound to the operator. There will be no formation of CAP cAMP complexes.

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