Question

For each of the merodiploid E. coli strains containing the lac operon alleles listed, indicate whether the strain is inducible, constitutive, or unable to expressβ-galactosidase and permease.

Genotype

1) I+ p+ o+ Z- Y+ /I+ oc Z+ Y+

2) I+ p+ o+ Z+ Y+ /I- p+ oc Z+ Y-

3) I- o+ Z- Y+ /I- p+ oc Z+ Y-

4) I+ p- o+ Z+ Y- /I- p+ oc Z- Y+

this is the problem with no further information

Answer 1

Answer: The structural genes Z, Y makes utilisation of lactose possible by synthesizing beta galactosidase (for breaking glycosidic bond in lactose) and permease ( for transporting lactose inside the cell) respectively.

if glucose is available lac operon will be switched off, and it will become active only if lactose is available in the medium. Normally when lactose is not there in the medium, an allosteric repressor synthesized by “ I “ gene of lac operon will bind to the operator region of the operon, obstructing the path of RNA polymerase required for transcription of the structural genes.

If lactose is available it binds to the repressor, and the repressor is no longer able to bind the operator region and the RNA polymerase will be able to transcribe the structural genes for lactose utilisation.

(There is second control where the cAMP produced in absence of glucose binds to CAP(catabolite activator protein) and this in turn binds to the lac operon resulting in expression of lac operon.)

If lactose and glucose both are there in the medium then E.coli will utilise glucose first and not lactose, as glucose is the preferred source of energy

In the given examples there are two lac operon, hence if one of the operon is having mutation in certain gene, it will be compensated by the product/activity of other normal gene

1) I+ p+ o+ Z- Y+ /I+ oc Z+ Y+

Explanation: The first operon would not be able to transcribe beta galactosidase (Z-), though all the other genes are normal. The second operon is showing operator with constitutive mutation that means the operator will not allow binding of repressor even in absence of lactose and hence beta galactosidase and permease will be transcribed even in absence of lactose.

The given strain will show constitutive expression of beta galactosidase and permease

2) I+ p+ o+ Z+ Y+ /I- p+ oc Z+ Y-

Explanation: In the first operon all the genes are normal, while the second operon is showing I- hence it will not produce the repressor, but repressor is a trans acting protein hence repressor synthesized by the first operon can bind the operator of the second operon but then operator of second operon is showing constitutive mutation hence it will not allow the repressor to bind and it will allow the transcription of beta galactosidase constitutively, but it has Y- mutation hence permease will not be produced by the second operon.

Hence transcription of beta galactosidase will be constitutive under the influence of second operon and transcription of permease will be inducible under the influence of first operon.

3) I- o+ Z- Y+ /I- p+ oc Z+ Y-

Explanation: The first operon would not be able to synthesize beta galactosidase and repressor because of the mutation Z- and I- . the second operon will not be able to synthesize repressor and permease because of mutation I- and Y-

As no repressor is synthesized by both the operon and the second operon is showing constitutive mutation in operator, the expression of structural genes will be constitutive

That is permease will be synthesized constitutively by the first operon and beta galactosidase will be synthesized constitutively by the second operon

4) I+ p- o+ Z+ Y- /I- p+ oc Z- Y+

Explanation: The first operon is not able to transcribe both the structural genes Z and Y because there is mutation in the promoter, hence RNA polymerase will not be able to initiate the transcription.(effect of no synthesis of permease because of Y- mutation need not be considered as the operon is non functional because of mutation P-).

The second operon is not able to transcribe repressor and beta galactosidase and it is also showing constitutive mutation in operator hence repressor will not be able to bind the operator.

Hence permease will be constitutively expressed by the second operon and beta galactosidase will be not be transcribed at all.

Note: in the third case you have not mentioned anything about the promoter gene in the first operon, hence it is considered as P+

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