Question

The lac operon in E. coli regulates the co-expression of three genes needed for the degradation of lactose (but absence of glucose) in the growth medium. These genes are not expressed when the bacteria has no lactose to feed on. How does the cell prevent expressing the lac operon in the absence of lactose? How does the presence of lactose activate the transcription of these three genes? (You do not need to explain the glucose level regulation.)

Answer 1

Cell prevent expressing the lac operon in the absence of lactose - The lac operon has a lac I gene that encodes a lac repressor which is always expressed unless a co-inducer(lactose in medium)attaches to it. The lac repressor functions to inhibit the production of the three genes by attaching to the operator of the operon in absence of lactose and this prevents the RNA polymerase to bind to promoter to initiate the transcription of the 3 genes.

Presence of lactose activate the transcription of these three genes - In presence of the lactose, the repressor molecule is bound to the lactose and this prevents the repressor to bind to the operator.When operator is free from repressor, the RNA polymerse binds to the promoter and the transcription of the three genes is initiated.