Question

Which of the following amino acids (given in single-letter symbol) present in the N-terminal region of histone proteins is involved in acetylation that influences genome expression?

• A. R
• B. K
• C. S
• D. T

Hint:

K = Lysine = Acetylation (Relaxed DNA $\rightarrow$ ON).
R = Arginine = Methylation (Usually OFF).
S/T = Serine/Threonine = Phosphorylation.
Remember: "Kindness (Acetylation) opens the door (Gene Expression)!"

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks to identify the specific amino acid residue on histone "tails" that undergoes acetylation, a key epigenetic modification that opens up chromatin structure for gene expression. The symbols are given in single-letter code.

Step 2: Detailed Explanation:

• Histone Modification: Histones are basic proteins around which DNA is wrapped. Their flexible N-terminal tails stick out and can be modified by enzymes.

• Acetylation: The enzyme Histone Acetyltransferase (HAT) adds an acetyl group from Acetyl-CoA to the $\epsilon$-amino group of Lysine residues.

• Single-Letter Codes:
- K stands for Lysine.
- R stands for Arginine.
- S stands for Serine.
- T stands for Threonine.

• Mechanism of Gene Activation: Lysine is positively charged, which creates a strong electrostatic attraction to the negatively charged DNA backbone. Acetylation neutralizes the positive charge of the Lysine. This weakens the histone-DNA interaction, causing the chromatin to relax (Euchromatin), allowing transcription factors to access the DNA.

• Significance: Histone acetylation (especially on H3 and H4 tails) is almost always associated with active gene transcription. Deacetylation (by HDACs) is associated with gene silencing.

Step 3: Final Answer:
The amino acid Lysine (K) is the primary target for acetylation in histone proteins.