Question at position 3 Using the nomenclature above, which partial diploid strain will produce β-galactosidase and transacetylase constitutively, but be unable to express permease?I = Lac repressor genep = the promotero = the operatorZ = the lacZ geneY= the lacY geneA = the lac A geneIsp+o+Z+Y+A+/I+p-ocZ-Y+A+I+p+ocZ+Y-A-/I+p+o+Z-Y+A+Isp+ocZ+Y-A+/I+p+o+Z-Y+A+I+p+o+Z+Y+A+/I+p-ocZ+Y-A+I+p+ocZ-Y+A+/I+p+o+Z+Y+A+

Question

Question at position 3 Using the nomenclature above, which partial diploid strain will produce β-galactosidase and transacetylase constitutively, but be unable to express permease?I = Lac repressor genep = the promotero = the operatorZ = the lacZ geneY= the lacY geneA = the lac A geneIsp+o+Z+Y+A+/I+p-ocZ-Y+A+I+p+ocZ+Y-A-/I+p+o+Z-Y+A+Isp+ocZ+Y-A+/I+p+o+Z-Y+A+I+p+o+Z+Y+A+/I+p-ocZ+Y-A+I+p+ocZ-Y+A+/I+p+o+Z+Y+A+

BlackTom AI · Accepted Answer

First, we need to parse what the genotype qualifiers mean for the Lac operon behavior. Key ideas: I indicates the Lac repressor gene, p is the promoter, o is the operator, Z encodes β-galactosidase, Y encodes permease, A encodes transacetylase. An operon that is constitutively expressed for Z and A but cannot express permease (Y-) typically involves an operator mutation (oc) that prevents repression by the LacI repressor, enabling Z and A transcription regardless of inducer presence, while a nonfunctional or deleted lacY would yield no permease activity. With that in mind, we evaluate each option one by one.

Option 1: Isp+o+Z+Y+A+/I+p-ocZ-Y+A+
- The use of o+ (intact operator) means normal repression can occur in the absence of inducer; this would not yield constitutive Z and A expression. Since the operator is not oc, repression can still occur, making constitutive Z and A unlikely. Additionally, the Y and A notation here is inconsistent with a clear Y- state for lack of permease. This combination fails to produce constitutive Z and A with Y-.

Option 2: I+p+ocZ+Y-A-/I+p+o+Z-Y+A+
- Here oc is present, which could drive constitutive expression of the Z-Y-A region, but the Y- indicates a nonfunctional permease. However, the rest of the arrangement shows a mixed use of oc with o+ and an overall architecture that does not clearly separate the repressor from constitutive Z and A in the way the question implies. The presence of oc in this context does not cleanly guarantee constitutive Z and A with Y-; the surrounding elements suggest ambiguity in how Z and A are actually transcribed relative to the operon, making this option unreliable for the described phenotype.

Option 3: Isp+ocZ+Y-A+/I+p+o+Z-Y+A+
- This option includes I and oc, which supports constitutive transcription of the lacZYA operon for Z and A when oc prevents repression. The Y- indicator shows lack of functional permease, aligning with 
constitutive Z and A expression but no permease activity. The promoter is p+ and the operator is o+ (not interfering with oc’s effect), which is compatible with a constitutive Z and A expression profile while inability to express permease remains. This aligns well with the target phenotype.

Option 4: I+p+o+Z+Y+A+/I+p-ocZ+Y-A+
- With p+o+ (functional promoter and operator) and an oc mutation present later, the interpretation becomes muddled: oc is unlikely to be in a position that yields the described constitutive Z and A expression with Y- given the sequence. Also, the combination of p- and oc suggests reduced or absent transcription initiation at times, which conflicts with constitutive expression of Z and A. Overall, this option does not cleanly realize the requested phenotype.

Option 5: I+p+ocZ-Y+A+/I+p+o+Z-Y+A+
- This arrangement shows oc in play, but the Z- marker indicates a nonfunctional β-galactosidase, contradicting the constitutive expression of Z. Therefore it cannot produce constitutive Z activity while maintaining Y- and A+ as described. The mismatch w.r.t. the Z signal makes this option incorrect.

Putting the pieces together, the combination that best matches: constitutive expression of β-galactosidase (Z+) and transacetylase (A+) regardless of induction, while permease (Y) is nonfunctional (Y-), is the one that features an operator mutation oc to lock Z and A transcription in the presence of an intact promoter context, along with Y- showing lack of permease activity. Among the provided choices, the line that embodies that pattern is the option containing Isp+ocZ+Y-A+/I+p+o+Z-Y+A+. This choice reconciles oc-driven constitutive Z and A expression with a nonfunctional Y.

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