Question

answer the following questions about lac operon:

a) if the lack repressor protein is bound to the operator, are the E.coli metabolized lactose or not? explain your reasoning.

b) If the lack repressor protein is bound to the operator and cellular glucose levels are low, is cAMP-CAP complex bound adjacent to the promotor, or not? explain your reasoning.

c) if the lack repressor protein is bound to the operator and cAMP - CAP is bound to the CAP site, are the bacteria metabolizing lactose, or not? explain your reasoning.

Answer 1

The general structure of lac operon is shown in below. RNA polymerase bind at the promoter. Repressor protein bind at operator. The cAMP-CAP complex bind at the CAP site.

a) If lac repressor protein is bound to the operator, RNA polymerase unable to transcribe the lac genes. As no transcription occurs, E. coli unable to metabolize lactose.

b) When the cellular glucose levels is low, cAMP is produced which help to bind CAP protein at the CAP binding site. As binding of cAMP-CAP complex is depend on the glucose levels & not on the repressor protein, cAMP-CAP complex will be bind adjacent to the promoter.

c) If lac repressor protein is bound to the operator & cAMP-CAP complex is bound to the CAP site, that means glucose level is low or absent & lactose is also absent. Though cAMP-CAP complex is bound to the CAP site as repressor protein is blocking the transcription of RNA polymerase by binding with the operator, no transcription occurs. In absence of transcription of lac genes bacteria will be unable to metabolize lactose.