Make predictions about whether the absorbance at 420nm will be “low”, “medium” o

Question

Make predictions about whether the absorbance at 420nm will be “low”, “medium” or “high” under each of the following conditions in each of these strains when incubated with ONPG. (3 points)

Oc is a mutation to the operator that prevents all proteins from binding to it.

LacIQ is a mutation in the promoter of LacI that leads to increased transcription from this gene.

LacUV is a mutation in the CAP binding site that leads to constitutive protein binding at that site.

Explanation / Answer

LacZ-Oc (neither): RNA polymerase cannot bind to the promoter that slightly overlapped by the operator, which usually occupied by a tetrameric repressor protein (I) in the absence of lactose. In the presence of lactose, repressor cannot bind to the operator, therefore, RNA pol can bind to promoter and transcription of structural genes will occur. Here, beta-galactosidase cannot be produced to hydrolyze ONPG because mutation not allowed any protein to bind with the operator.

LaclQ, in which increased synthesis of repressor protein can be observed that binds to the operator and prevents the transcription of structural genes and beta-galactosidase cannot be produced. Lactose (high), since, it inhibits the repressor binding with the operator. Therefore, beta-galactosidase will produce and hydrolyze ONPG and give maximum absorbance.

Lac UV, in which CAP binding site of the promoter is constitutive so that in the presence of lactose we can find high absorbance and in the presence of glucose we can see low absorbance. Glucose levels associated with low cAMP, therefore, CAP binding protein is inactive that cannot bind with its site on the promoter.

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