Genes and products of the lac operon (E. coli) — z, y, and a In bacterial gene regulation, the lac operon contains three structural genes (z, y, and a). What do these genes encode, respectively?
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Aβ-galactosidase, lactose permease, and thiogalactoside transacetylase
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Bβ-galactosidase and lactose permease only (no third gene)
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Cβ-galactosidase only (no permease or transacetylase)
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DRegulatory proteins lacI and CAP (catabolite activator protein)
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EGalactokinase, galactose permease, and aldolase
Answer
Correct Answer: β-galactosidase, lactose permease, and thiogalactoside transacetylase
Explanation
Introduction:The lac operon of Escherichia coli is a classic model of transcriptional regulation. It enables lactose utilization when glucose is scarce and lactose is available. Knowing the gene products of lacZ, lacY, and lacA is foundational.
Given Data / Assumptions:
- Structural genes: lacZ, lacY, lacA (z, y, a).
- Regulatory elements (lacI repressor, promoter, operator, CAP site) are distinct from the structural genes.
- Functional roles relate to lactose metabolism and transport.
Concept / Approach:
lacZ encodes β-galactosidase, which hydrolyzes lactose into glucose and galactose and can isomerize lactose analogs. lacY encodes lactose permease, a membrane symporter importing lactose. lacA encodes thiogalactoside transacetylase, which acetylates certain galactosides (a detox role).
Step-by-Step Solution:
Map z → β-galactosidase, y → lactose permease, a → thiogalactoside transacetylase.Exclude regulatory proteins (lacI, CAP) from the structural gene list.Confirm that all three enzymes together support lactose uptake and catabolism.Select the option listing all three correctly.Verification / Alternative check:
Genetic mapping and complementation tests historically defined z, y, and a functions; biochemical assays confirmed corresponding enzyme activities.
Why Other Options Are Wrong:
Options B and C omit one or two genes. Option D lists regulatory proteins, not the structural gene products. Option E substitutes unrelated enzymes and even the wrong sugar focus.
Common Pitfalls:
Confusing lactose permease with “galactose” permease, or thinking lacA is essential for lactose hydrolysis (it is accessory).
Final Answer:
β-galactosidase, lactose permease, and thiogalactoside transacetylase