Suppose the lac operon partial diploid c a p ? I + P + O + Z ? Y +/ c a p + I ?

Question

Suppose the lac operon partial diploid

cap? I+ P+ O+ Z? Y+/ cap+ I? P+ O+ Z+ Y?

is grown. (cap- indicates that the CAP-cAMP binding site is not functional, and the copy of the operon will not be transcribed at levels that support growth.) In order to grow on lactose media, the strain needs to produce both functional beta-galactosidase (encoded by lacZ) and permease (encoded by lacY).

Part A: Will this partial diploid strain grow on a lactose medium? Yes or no?

Part B: Is transcription of beta-galactosidase inducible, constitutive, or noninducible?

Part C: Is transcription of permease inducible, constitutive, or noninducible?

Explanation / Answer

A. Yes. Lactose acts as an inducer. Though one of the chromosome lack the functional inducer, it is still under repression of the second one (trans). Thus, in the presence of lactose, the inhibition is removed and gene transcription occurs.

B. Inducible. Though absence of CAP leads to absence of CAP-cAMP mediated positive control, the operon is under the negative control of repressor protein binding. Hence, only in the presence of lactose, the operon is expressed, and hence, it is inducible. (However, notice that CAP is not absent, it is still present in one of the chromosomes in active form).

C. Inducible.

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