Question at position 13 How are the trpE and trpC genes regulated for an E. coli strain with the genotype: R− P+ o+ trpE+ trpC- / R+ P+ o+ trpE- trpC+ R = repressor gene o = operator for the trp operon P = promoter trpE− and trpC− are null (loss-of-function) mutations no expression of trpE or trpCtrpE is constitutively expressed, trpC is fully repressibleconstitutive expression of both trpE and trpCtrpE is partially repressible, trpC is fully repressibletrpE and trpC are repressibleClear my selection

Question

Question at position 13 How are the trpE and trpC genes regulated for an E. coli strain with the genotype: R− P+ o+ trpE+ trpC- / R+ P+ o+ trpE- trpC+ R = repressor gene o = operator for the trp operon P = promoter trpE− and trpC− are null (loss-of-function) mutations no expression of trpE or trpCtrpE is constitutively expressed, trpC is fully repressibleconstitutive expression of both trpE and trpCtrpE is partially repressible, trpC is fully repressibletrpE and trpC are repressibleClear my selection

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Question restatement: We are asked to determine how the trpE and trpC genes are regulated in an E. coli strain with the genotype: R− P+ o+ trpE+ trpC- / R+ P+ o+ trpE- trpC+ where R is the repressor, o is the operator, P is the promoter, and trpE− and trpC− denote loss-of-function mutations. Answer options: no expression of trpE or trpC; trpE is constitutively expressed, trpC is fully repressible; constitutive expression of both trpE and trpC; trpE is partially repressible, trpC is fully repressible; trpE and trpC are repressible.

Option 1: no expression of trpE or trpC. This would imply complete failure to transcribe or translate both genes under all conditions. However, the trp operon is structured so that transcription can occur and be regulated by the repressor–corepressor complex; the presence of functional promoter and operator suggests that transcription is possible when not repressed. Moreover, the genotype includes a functional repressor allele on one chromosome and a nonfunctional repressor on the other, making this blanket “no expression” scenario inconsistent with canonical operon regulation.

Option 2: trpE is constitutively expressed, trpC is fully repressible. If expression were constitutive for trpE, it would suggest separate, independent regulation for trpE. In the trp operon, trpE and trpC are part of a single polycistronic transcriptional unit controlled by the same repressor-operator system; thus they are co-regulated rather than independently regulated. Therefore this split pattern (constitutive for trpE only) is not consistent with operon-wide control.

Option 3: constitutive expression of both trpE and trpC. Constitutive expression would mean no repression occurs under any conditions. But the trp operon is a classic repressible operon: when tryptophan (the corepressor) is present, the trp repressor–corepressor complex binds the operator and inhibits transcription. Even with some nonfunctional alleles present, the regulatory mechanism itself is designed to be repressible, not constitutive.

Option 4: trpE is partially repressible, trpC is fully repressible. This option proposes different degrees of repression for the two genes. In an operon, the genes are typically co-regulated as a unit; partial versus full repression of individual genes would imply differential regulation within the same operon, which is not characteristic of the classical trp operon architecture. Additionally, the model organism’s operon regulation tends to act uniformly across the operon when the repressor-operator complex is engaged.

Option 5: trpE and trpC are repressible. This aligns with the well-established concept that the trp operon is repressible: in the presence of tryptophan, the repressor–corepressor complex binds the operator to reduce transcription of the entire operon, thereby repressing both trpE and trpC together as part of the same transcriptional unit. Even though one chromosome carries a loss-of-function mutation and the other carries the complementary loss-of-function allele, the regulatory scheme governing expression from the operon as a whole remains repressible by the tryptophan–repressor mechanism.

In summary, evaluating the logic of operon regulation and the nature of the trp operon as a whole, the option that best fits the canonical mechanism is that trpE and trpC are repressible. Each incorrect choice either contradicts the unified, operon-wide regulation model, introduces inappropriate independence between genes, or mischaracterizes the expression pattern under repression.

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