Histone Modifications and Variants
Histones are subject to a wide variety of posttranslational modifications including but not limited to, lysine acetylation, lysine and arginine methylation, serine and threonine phosphorylation, and lysine ubiquitination and sumoylation (Vasquero 2003). These modifications occur primarily within the histone amino-terminal tails protruding from the surface of the nucleosome as well as on the globular core region (Cosgrove 2004).
Histone modifications are proposed to affect chromosome function through at least two distinct mechanisms. The first mechanism suggests modifications may alter the electrostatic charge of the histone resulting in a structural change in histones or their binding to DNA. The second mechanism proposes that these modifications are binding sites for protein recognition modules, such as the bromodomains or chromodomains, that recognize acetylated lysines or methylated lysine, respectively.
The existence of these modifications and recognition modules led to a well established “histone code” hypothesis proposed by Strahl and Allis (2000). Overall, posttranslational modifications of histones create an epigenetic mechanism for the regulation of a variety of normal and disease-related processes.
Histone Variant Composition
A histone variant is distinguishable from a core (canonical) histone by a small number of amino acid changes. It is believed that the incorporation of histone variants in place of a core histone contributes to marking regions of the chromatin for specialized functions and to reversing the effects of histone methyltransferases (histone replacement model).
Current topics in chromatin research
Interesting Review Articles
Bannister, AJ., and Kouzarides, T. Reversing Histone Methlyation. Nature. 2005. 436: 1103-1106.
Jenuwein T. and Alis, CD. Translating the Histone Code. Science. 2001; 293; 1074-1080.
Kimmins, S. and Sassone-Corsi, P. Chromatin Remodelling and Epigenetic Features of Germ Cells. Nature. 2005; 434: 583-589.
Kamakaka, RT. and Biggins, S. Histone Variants: Deviants? Genes & Development. 2005; 19: 295-310.
Strahl, BD. and Allis, CD. The Language of Covalent Histone Modifications, Nature. 2000; 403: 41-45.
Vasquero, A. et. al. The Constantly Changing Face of Chromatin. Science of Aging Knowledge Environment. 2003; RE4.