Question

5. Jacob, Monod, and Pardee used various E.coli mutants to help determine how the lac operon is regulated. The descriptions of some of the mutants are listed. For each E. coli mutant determine if the lac operon would be on or off in a lactose-only environment. laci mutant: This mutant produces a super repressor that is not inactivated by allolactose. It will still stick. Operater will be off, lacO mutant: This mutant has an altered operator sequence so that the repressor protein is no longer able to bind the operator sequence. Operon is constant! • lacP mutant: This mutant has an altered promoter sequence so that RNA polymerase cannot bind. Is it never sticks to DNA, it will never transcribe. It is off forever. 6. The lac operon is not only regulated by the presence or lactose. Glucose is always the preferred sugar since it is a monosaccharide and is easier for a bacterium to use compared to lactose which is a disaccharide. Therefore, the presence of glucose also regulates the lac operon but in a very different way. Look up and describe the relationship between glucose supply, CAMP, and CAP.

Answer 1

Answer-

1. lac Is mutant: This mutant produces a super repressor that is not inactivated by allolactose

= This super repressor either can't bind to the inducer or repressor can bind to the inducer but it can not undergo confirmation change so that repressor gets removed from the operator. So that allolactose is inducer that can not inactive repressor so that repressor will not be removed from operator and operon will be shut off in presence of lactose only environment.

2. lac Oc mutant: This mutant mutant altered operator sequence so that the repressor protein is no longer able to bind the operator sequence

= in this mutant operator sequence of lac operon is mutated so that repressor protein cannot bind to the operator and if repressor can not bind to operator it cannot shut off operon. If repressor bound to operator then operon is off and I repressor not bound to operator then operon is on.

This lacOc mutant will be always producing enzymes for lactose utilization even though lactose is absent in environment. Here operon is continuously on and transcription can not be shut off in presence of lactose.

3. lac P- mutant: This mutant has an altered promoter sequence so that RNA polymerase cannot bind

=Promoter is the sequence where RNA polymerase binds and initiates transcription. In this mutant the promotor has altered sequence so that RNA polymerase cannot bind to it and if RNA polymerase can not bind to promoter transcription of lac operon genes is not possible. Even though lactose is present in environment lac operon is off due to defective promoter sequence.