HDAC inhibitor drugs like trichostatin A cause widespread, persistent gene activation. What is the direct mechanism explaining this effect?
AThey activate HATs, which add acetyl groups to histone lysines and directly open chromatin
BThey block histone deacetylases, preventing removal of acetyl groups; without deacetylation, the weakened histone-DNA contacts persist and chromatin remains accessible
CThey methylate histone H3K4, a mark associated with active transcription, which opens chromatin
DThey increase acetyl-CoA availability in the nucleus, providing more substrate for ongoing acetylation
HDACs continuously remove acetyl groups from histone lysines, restoring positive charges that re-tighten the electrostatic grip on DNA. Blocking HDACs prevents this removal, so acetyl marks accumulate and the weakened histone-DNA contacts persist — keeping chromatin accessible for transcription. The effect is broad because many genes are in a dynamic equilibrium of acetylation/deacetylation, and tipping this balance toward acetylation broadly activates transcription. This is also why HDAC inhibitors are explored as cancer therapeutics.
Question 2 Multiple Choice
Histone acetylation weakens histone-DNA contacts and promotes chromatin opening. What is the fundamental mechanism by which adding an acetyl group to lysine achieves this?
AThe acetyl group adds bulk to the histone tail, physically pushing DNA away from the nucleosome
BAcetylation targets lysine residues for proteasomal degradation, reducing the number of histones available to compact DNA
CAcetylation neutralizes the positive charge on lysine, reducing the electrostatic attraction between histone tails and the negatively charged DNA backbone
DAcetylation recruits chromatin remodeling complexes that use ATP to reposition nucleosomes
The mechanism is electrostatic. Lysine's amino group carries a positive charge at physiological pH; the DNA phosphate backbone is strongly negative. This charge complementarity holds DNA tightly wrapped around the histone octamer. Acetylation transfers an acetyl group onto the lysine amino group, neutralizing the positive charge and reducing the electrostatic attraction. The DNA loosens. Option D describes what happens next — chromatin remodeling complexes often complete the opening — but acetylation itself works through charge neutralization, not recruitment alone.
Question 3 True / False
The rapid reversibility of histone acetylation — occurring within minutes — makes it suited for transient gene responses to immediate cellular signals, in contrast to DNA methylation, which can persist through multiple cell divisions.
TTrue
FFalse
Answer: True
This contrast is the key insight for understanding why different epigenetic mechanisms are used in different contexts. HATs and HDACs operate rapidly, creating a dynamic equilibrium that allows genes to be switched on and off in response to fast-changing signals like growth factors or stress. DNA methylation, once established, is propagated by maintenance methyltransferases during replication and persists indefinitely — it is more like a permanent circuit rewiring. Acetylation is the fast-response switch; methylation is the developmental lock.
Question 4 True / False
Histone acetyltransferases (HATs) directly open chromatin by repositioning nucleosomes, making underlying DNA sequences accessible for transcription.
TTrue
FFalse
Answer: False
This conflates two separate steps. HATs add acetyl groups to histone lysines, neutralizing positive charges and weakening histone-DNA contacts — but this is not the same as fully opening chromatin. Nucleosome repositioning typically requires ATP-dependent chromatin remodeling complexes (like SWI/SNF or ISWI), which use energy to slide or eject nucleosomes. HATs weaken the electrostatic grip; remodelers do the physical repositioning. This is the key misconception noted in the topic: acetylation is necessary but often not sufficient for full chromatin opening.
Question 5 Short Answer
Why does blocking HDACs with drugs like trichostatin A cause broad and persistent gene activation rather than selective activation of a few target genes?
Think about your answer, then reveal below.
Model answer: Histone acetylation is maintained by a continuous competition between HATs (which add acetyl marks) and HDACs (which remove them). Many genes across the genome exist in a dynamic equilibrium — their acetylation state is not fixed but constantly renewed. When HDACs are blocked, acetyl marks accumulate on histones genome-wide because the removal reaction is halted while HATs continue to add marks. This tips the balance toward open chromatin broadly, activating many genes simultaneously. The effect is persistent because it continues until the drug is removed or degraded.
This question reveals the dynamic, equilibrium nature of histone acetylation — it is not a one-time on/off switch but a constant cycle of addition and removal. The therapeutic implications are significant: HDAC inhibitors cannot target specific genes, which is both a limitation (broad effects cause toxicity) and sometimes a feature (broad activation of tumor suppressor genes in cancer cells).