How many proteins are translated from the E. coli lac operon structural genes (lacZ, lacY, lacA)?

Introduction / Context:
The lac operon is a textbook example of polycistronic gene organization in bacteria. A single mRNA encodes multiple proteins that collaborate in lactose uptake and metabolism.

Given Data / Assumptions:

• Structural genes: lacZ, lacY, lacA.
• Prokaryotic mRNAs can be polycistronic (multiple open reading frames).
• Each cistron is translated to yield a distinct protein product.

Concept / Approach:
lacZ encodes beta-galactosidase, lacY encodes lactose permease, and lacA encodes thiogalactoside transacetylase. Together they amount to three translated proteins from one operon transcript.

Step-by-Step Solution:
Count the structural genes in the lac operon (Z, Y, A).Recognize polycistronic translation in bacteria.Select the total number of protein products: 3.

Verification / Alternative check:
Mutational analyses and biochemical assays confirm three distinct enzymatic activities corresponding to the three gene products.

Why Other Options Are Wrong:

• 1 or 2: Underestimates the number of structural genes.
• 4 or 5: Overestimates beyond known structural genes of the classical lac operon.

Common Pitfalls:
Confusing regulation components (lacI repressor) with the structural genes on the polycistronic mRNA.

Final Answer:
3.