CASE FILES: BIOCHEMISTRY
Acetylation and deacetylation of lysine residues on histone proteins
provide one mechanism by which transcription can be activated or
repressed. Which one of the histone proteins is least likely to partici-
pate in this process?
A nsw ers
D. Ciprofloxacin is a fluoroquinolone antibiotic that will inhibit the
strand-cutting function of A subunit of DNA gyrase, a bacterial topoi-
somerase II that introduces negative supercoils ahead of the replication
fork. This disrupts DNA replication and repair, transcription, bacterial
chromosome separation, and other bacterial processes involving DNA.
At much higher concentrations, the type II topoisomerases of eukary-
otic cells can be inhibited.
B. When lysine residues in the ^-terminal portion of histones are
acetylated, it decreases the positive charge of the histone proteins and
weakens the interaction between the histones and the DNA. As a
result, the nucleosomes are “opened up” and lead to gene activation.
A. Nucleosomes are disk shaped particles that consist of a core of his-
tone protein around which DNA is wrapped. A short linker region of
DNA joins nucleosomes. The core of the nucleosomes is made up of
two copies each of histones H2A, H2B, H3, and H4. These histone
proteins have ^-terminal “tails” that contain lysine residues that can
be reversibly acetylated, affecting the electrostatic interaction of the
DNA with the histones. Histone H1 is not part of the nucleosomes
core, therefore acetylation would likely not affect the protein-DNA
interaction of the nucleosomes.