How does the lac repressor affect lac operon expression?

The lac repressor plays a crucial role in the regulation of the lac operon by inhibiting expression, particularly in environments where lactose is scarce. When lactose levels are low, the lac repressor binds to the operator region of the lac operon, blocking RNA polymerase from transcribing the genes necessary for lactose metabolism. This effectively prevents the expression of the operon, conserving cellular resources when lactose is not available.

In contrast, when lactose is present in sufficient quantities, it acts as an inducer that binds to the lac repressor, causing a conformational change that releases the repressor from the operator. This allows RNA polymerase to access the promoter and initiate transcription of the structural genes involved in lactose breakdown. Therefore, the correct understanding of the repressor's function highlights how it protects cells from unnecessary gene expression in response to low lactose availability.