For this question, focus on four parts: I, the gene that produces the repressor

Question

For this question, focus on four parts:

I, the gene that produces the repressor protein;

P, the promoter region where RNA polymerase binds to initiate transcription;

O, the Operator site, where the repressor binds. When the repressor binds, it prevents transcription; and

ZYA, the three structural genes, all of which are required for the successful utilization of lactose as a food source.

In the left column of the table below, we indicate the genetic composition at the lac operon of particular strains of E. coli cells. A "+" (plus sign) following the letter indicates the wild-type allele (or normally functioning version of the gene). For I, P, and ZYA, a "-" (negative sign) following the letter indicates the mutant allele (or non-functional version of the gene). For the O allele, a "C" indicates the mutant or constitutive allele. It is not functional in that the repressor cannot bind, but it is functional in that transcription can still occur.

Given the conditions stated in each column, please indicate for each genotype whether lac mRNA is produced and at what level by typing in one of the following words in each cell in the table:

"Abundant"

"Minimal"

"None"

The completely wild-type genotype has already been filled in.

Cellular conditions: Genotype Glucose present (low cAMP) and no lactose Glucose present (low cAMP) and lactose present No glucose (high cAMP) and lactose present I+ P+ O+ ZYA+ None Minimal Abundant I- P+ O+ ZYA+ I+ P- O+ ZYA+ I+ P+ Oc ZYA+ I+ P+ O+ ZYA-

Explanation / Answer

In first case since repressor protein is not produced, RNA Polymerase can easily bind to promoter and carry the transcripton. But when glucose is present, Catabolite activator protein (CAP) cannot bind the cap site. CAP helps in the binding of RNA polymerase. Thus minimal transcription occurs in the presence of glucose.

In second case, since the mutant allele is in the promoter region, RNA polymerase cannot bind and thus no transcription.

In third case, mutant allele is in the operator region. Repressor can no longer bind, thus transcription will occur minimally in the presence of glucose and abundantly in the absence of glucose.

In the fourth case, since the structural genes itself are mutant, lac mRNA is not produced.

I-P+O+ZYA+ MINIMAL MINIMAL ABUNDANT I+P-O+ZYA+ NONE NONE NONE I+P+Oc ZYA+ MINIMAL MINIMAL ABUNDANT I+P+O+ZYA- NONE NONE NONE

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