Histone code

The Histone Code is hypothesized to be an code consisting of covalent histone tail modifications.

Histones main role is in associating with DNA to form nucleosomes, which themselves bundle to form chromatin fibers. They are globular proteins with a flexible N-terminus (taken to be the tail) that protrudes from the nucleosome. The chromatin structure plays an important role in regulation of gene expression, while the tail modifications play an important role in the chromain structure.

The Hypothesis

The hypothesis predicts that:

• Tail modification affects the interaction between the chromatin and associated proteins.
• Modifications on the same or different tails may be interdependant (ie. the "code" depends on combinations of modifications).
• The characteristics of such chromatin organisations as euchromatin and heterochromatin are results of tail modifications.

Modifications

Possible modifications to the tails are:

• Acetylation - by HAT (Histone Acetyl Transferase); deactylation - by HDAC (Histone Deacetlyase)

Deacetylation allows tight arrangement of chromatin, preventing gene expression, while acetlyation may occur to open up the chromatin.

• Methylation
• Phosphorylation