Histones are the major proteins involved DNA packaging in eukaryotes.
Histones are very basic proteins; about 25% of their amino acid are lysine or arginine so histones have a large number of positively charged amino acid side chains. The positive charge helps histone to bind to DNA and play a crucial role in DNA packaging.
The amino acid sequences of histones are highly conserved in evolution. The most conserved regions are H3 and H4. H1 is the least conserved histone.
In sperm heads, DNA is particularly highly condensed and here the histones are replaced with small basic proteins called protamines.
There are five main types of histones called H1, H2A, H2B, H3 and H4. The five histone are grouped into two categories: Nucleosomal histones and H1 histones.
Types of Histones: Nucleosomal histones and H1 histones
1. Nucleosomal histones
- comprises of H2A, H2B, H3 and H4.
- Each one made up of approx. 102-135 amino acids.
- These proteins are highly conserved during evolution.
- H3 and H4 are the most conserved.
- They are responsible for coiling of DNA into nucleosomes.
2. H1 histones
- They are loosely associated with core particle and comprises of approx 200 amino acids.
- least conserved and tissue specific.
- They are present one molecule per 200 base pairs and are loosely associated with DNA.
- Only one H1 molecule is preset with each nucleosome.
- H1 histones regulate transcription by repressing specific DNA segments. So that, these segments are unable to transcribe.
- Functions: H1 histones are primarily involved in packaging of nucleosomes into 30nm fibre.
Characteristics of histone proteins
- MW-21,130 / No. of amino acids-223
- MW-13,960 / No. of amino acids-129
- MW-13,774 / No. of amino acids-125
- MW-15,273 / No. of amino acids-135
- MW-11,236 / No. of amino acids-102
Functions of Histones
- These serve as structural element and help in coiling and packaging a along DNA molecules.
- These cover or repress specific segments of DNA so that these segments are unable to transcribe. Their transcription is possible only when histones are removed in response to certain signals.