Histone Modification: Acetylation n Methylation

1. Histone Modifications: Acetylation and Methylation Somanna A. N.

2. Complexes of DNA and protein form Eukaryotic chromosmes . These proteins are divided into two classes: Histones and nonhistone chromosomal proteins. Histones are responsible for most basic level of chromosome packing – Nucleosome , a protein-DNA complex.

3. Nucleosome DNA wound around a histone core Made of ‘ nucleosome core particle ’ and ‘ linker DNA ’

4. Histone Core An octamer of 4 histone proteins- H2A, H2B, H3 and H4. Each histone protein has a structured domain, ‘ Histone Fold’ and unstructured ‘N- terminal tail’.

5. Histone tails are involved in the 30-nm fibre formation by forming H bonds with adjacent nucleosome tails. Histone tails provide site for covalent modifications- Acetylation , Methylation , Phosphorylation , etc. Such modifications determine the interaction of histone with other proteins, which may in turn regulate chromatin structure, and transcription , etc.

6. Modifications of N- terminal tails reduce ability of nucleosome arrays to form repressive structures and creating sites that can recruit other proteins. It has been proposed that these modifications result in a ‘code’ which can be read by proteins involved in gene expression and other DNA transcations .

7. Acetylation It is the introduction of an Acetyl functional group to the Lysine amino acid of the histone tail. These reactions are catalyzed by enzymes with "histone acetyltransferase" (HAT) or "histone deacetylase" (HDAC) activity.

8. Acetylation removes + ve charge of the histone tail, reducing affinity for the – ve charged phosphate groups of DNA It also reduces affinity of tail for adjacent nucleosomes , thus affecting ability of nucleosome arrays to form more repressive higher-ordered chromatin structures. Also, acetylation is involved in nucleosome assembly and interaction of histone with other regulatory proteins, creating a transcription permissive environment.

9. Modification of histone tails by acetylation is known to increase the access of transcription factors to DNA through structural changes in nucleosomes or nucleosomal arrays. Acetylated histones are also specifically recognized by other proteins. The bromodomain , found in transcription factors and HATs allows for the preferential recognition of histone tails when they are acetylated at specific lysine residues 

10. Methylation It is the introduction of an Methyl functional group to Lysine or Arginine of the histone tail. These reactions are catalyzed by enzymes with "histone methyltransferase ” ‘ Arg ’ can be methylated once or twice, and ‘Lys’ once, twice of trice.

11. Methylation does not neutralize charge but recruit silencing or regulatory proteins that bind methylated histones . Chromodomain containing proteins interact with methylated histone tails.

12. Methylation generally associated with transcription repression, but specific methylations result in activation. They can loosen the tail allowing transcription factors to access DNA or encompass the tails around DNA restricting access

13. Histone Code The histone tail modifications can occur in varoius combinations, many of which have specific meaning- HISTONE CODE. This code is read by specific protein complexes which contain protein modules which recognise specefic histone marks and bind to them .