E coli cells are growing in a medium containing lactose but

E. coli cells are growing in a medium containing lactose but no glucose. How would a mutation that prevents binding of CAP to its binding site near the Lac promoter affect expression of the Lac operon?

A) increase expression of the lac operon

B) decrease expression of the lac operon

C) not change expression of the lac operon

Solution

B) decrease expression of the lac operon.

The CAP binding site is a positive regulatory site that is bound by catabolite activator protein (CAP). When CAP is bound to this site, it promotes transcription by helping RNA polymerase bind to the promoter. CAP binds to a region of DNA just before the lac operon promoter and helps RNA polymerase attach to the promoter, driving high levels of transcription.

Therefore when CAP is not bound to its binding site, expression of lac operon will decrease.