31. For each of the E. coli strains containing the lac operon alleles listed, in

Question

31. For each of the E. coli strains containing the lac operon alleles listed, indicate whether the strain is inducible, constitutive, or unable to express B-galactosidase and permease. Provide an explanation for your prediction. (16 pts) A. O* Z* Y B. I' O* CRP Z* Y A. Constitutive expression of lacz and no expression of lacr. In this strain, repressor is mutated resulting constitutive production of lacz and lacr mRNA. However, lacr is mutated so permease is not produced. B. Nobasal expression lacz and lacY. CRP is a positive regulator of lac operon expression. The lack of CRP results no expression of lac operon C. lacz is constitutive; lacris constitutive. In this merozygote,one of the operators adjacent to wild type lacz and lacr is mutated and does not bind to the repressor. This results in constitutive expression of both genes. D. No expression of lacz: no expression of l In this merozygote, one of the repressors is such a way that the mutant protein binds the operator but can't bind the inducer. Since repressors can act in trans this strain will not have any expression of lacz and lacr

Explanation / Answer

A: The operon has mutant I gene. The I gene codes for the repressor protein which in turn prevents the expression of operon in absence of lactose by binding to the operator region. Since the mutant I gene would not produce the repressor protein or will encode the non functional repressor protein; the operon will keep on expressing even in absence of lactose i.e. constitutive expression.
B: The operon has mutant CRP gene. The CRP genes causes expression of lac operon through the positive regulation. Presence of mutant CRP gene would lead to no expression of operon even in presence of lactose.
C: The operon is constitutive (Oc) and mutant I gene is present which would cause the constitutive expression of operon even in absence of lactose.
D: The Is gene codes for repressor protein that binds to operator but not to the lactose. Binding of repressor to lactose causes its deactivation and exposes the promoter for the binding of RNA polymerase. The mutant repressor would lead to expression of the operon.

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