Question

The -10 box is the nearest upstream sequence to which of the following gene of lac operon of E. coli?

• A. lac Y
• B. lac Z
• C. lac A
• D. lac I

Hint:

Remember the sequence: P-O-Z-Y-A.
Z is for $\beta$-galactosidase.
Y is for Permease.
A is for Transacetylase.
The "Z" is always first in line after the "switch" (operator/promoter)!

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks to identify the structural gene of the lac operon that is located most immediately downstream of the promoter elements, specifically the -10 box (Pribnow box).
The lac operon is a classic model of gene regulation in prokaryotes (E. coli) used to control the metabolism of lactose.

Step 2: Detailed Explanation:

• Structure of the Lac Operon: The lac operon consists of a regulatory gene (lacI), a promoter (P), an operator (O), and three structural genes: lacZ, lacY, and lacA.

• Promoter Region: The promoter contains two essential conserved sequences recognized by the RNA polymerase sigma factor: the -35 box and the -10 box (Pribnow box). These are located upstream of the transcription start site (+1).

• Linear Arrangement: The transcription of the operon produces a single polycistronic mRNA. The order of genes from the promoter moving downstream is lacZ $\rightarrow$ lacY $\rightarrow$ lacA.

• Proximity: Because lacZ (encoding $\beta$-galactosidase) is the first structural gene in the sequence, it is the one nearest to the promoter's regulatory elements like the -10 box.

• Analysis of lacI: Although lacI is part of the overall lac system, it is a regulatory gene located far upstream and has its own independent promoter. It is not considered part of the polycistronic structural unit.

Step 3: Final Answer:
The lacZ gene is the first gene of the lac operon and is therefore the closest gene downstream of the -10 promoter sequence.