LAC OPERON STRUCTURE

The L A C operon has a region labeled Regulatory Gene that contains loci P subscript 1 and l a c 1. The L A C operon has a 2nd, separate region that contains five loci, in order from left to right on the DNA strand, P subscript l a c, operator, l a c Z, l a c Y, l a c A.

The functions of the loci of the lac operon shown in the diagram are described in the table below.

Functions of the loci of the lac operon
Locus Function
PI Attachment site for RNA polymerase
lacI Encodes a repressor protein that prevents transcription of the structural genes of the lac operon
Plac Attachment site for RNA polymerase
Operator Binding site for the repressor protein
lacZ Encodes beta-galactosidase, the enzyme that digests lactose to glucose and galactose
lacY Encodes lactose permease, the channel through which lactose moves into the cell
lacA Encodes galactoside transacetylase

The diagram above represents a segment of the E. coli chromosome that contains the lacI gene and part of the lac operon, a coordinately regulated set of genes that are required for the metabolism of lactose. The presence of lactose, which causes the repressor to be released from the operator, results in increased transcription of the lac operon.

Which of the following is the most likely consequence of a mutation at the operator locus that prevents binding of the repressor protein?

Question

LAC OPERON STRUCTURE

The L A C operon has a region labeled Regulatory Gene that contains loci P subscript 1 and l a c 1. The L A C operon has a 2nd, separate region that contains five loci, in order from left to right on the DNA strand, P subscript l a c, operator, l a c Z, l a c Y, l a c A.

The functions of the loci of the lac operon shown in the diagram are described in the table below.

Functions of the loci of the lac operon
Locus	Function
PI	Attachment site for RNA polymerase
lacI	Encodes a repressor protein that prevents transcription of the structural genes of the lac operon
Plac	Attachment site for RNA polymerase
Operator	Binding site for the repressor protein
lacZ	Encodes beta-galactosidase, the enzyme that digests lactose to glucose and galactose
lacY	Encodes lactose permease, the channel through which lactose moves into the cell
lacA	Encodes galactoside transacetylase

The diagram above represents a segment of the E. coli chromosome that contains the lacI gene and part of the lac operon, a coordinately regulated set of genes that are required for the metabolism of lactose. The presence of lactose, which causes the repressor to be released from the operator, results in increased transcription of the lac operon.

Which of the following is the most likely consequence of a mutation at the operator locus that prevents binding of the repressor protein?

BlackTom AI · Accepted Answer

When the operator cannot bind the lac repressor, the structural genes of the operon are expressed constitutively. Key concepts: lacI encodes a repressor protein that binds the operator and physically prevents RNA polymerase from transcribing lacZ, lacY, and lacA in the absence of lactose. When lactose (actually allolactose) is present it binds the repressor, alters its shape, and causes it to release the operator so transcription can proceed.

Step-by-step reasoning:
1. In wild-type cells without lactose: lacI-produced repressor binds the operator sequence; this blocks RNA polymerase from initiating transcription at Plac, so lacZ (beta-galactosidase), lacY, and lacA are not produced.
2. In wild-type cells with lactose: the repressor is inactivated by the inducer, releases the operator, RNA polymerase transcribes the structural genes, and beta-galactosidase is synthesized.
3. If the operator is mutated so the repressor can no longer bind, the blocking mechanism is lost regardless of lactose presence. RNA polymerase can bind Plac and transcribe continuously (constitutive expression).
4. Therefore beta-galactosidase (product of lacZ) will be produced even when lactose is absent.

Why the correct answer is correct: Loss of repressor binding removes negative control; transcription proceeds without the inducer, so beta-galactosidase appears constitutively.

Why the other options are incorrect:
- "Expression ... will be repressed, even in the presence of lactose." This is the opposite of the expected effect; a nonbinding operator cannot mediate repression.
- "RNA polymerase will attach at Plac, but transcription will be blocked." If the repressor cannot bind, there is no block; transcription is allowed.
- "The operator locus will code for a different protein..." The operator is a noncoding regulatory DNA site; it does not code for protein, so a mutation there does not create a repressor-like protein that prevents transcription.

Conclusion: Beta-galactosidase will be produced even in the absence of lactose, which matches the provided correct answer.

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