Histone Modifications-Marks for Gene Expression?
During early embryonic development stable patterns of gene expression have to be established and maintained over several cell generations. In higher eukaryotes the activity of a particular gene has been shown to depend on a coordinated action of several classes of proteins and protein complexes ranging from chromatin remodelling machines to basal transcriptions factors and eventually the RNA polymerase itself. It is less clear how such a activated or repressed state is then maintained during subsequent cell divisions. Many transcription factors involved in gene activation or repression get transiently displaced from their binding sites during mitosis or disappear completely. Over the last few years, chromatin has been shown to play an important role in regulating gene expression and establishing stable patterns of gene activity in response to incoming signals (Cheung et al., 2000; Wolffe, 1998). The fundamental unit of chromatin, the nucleosome consists of an octamer made up by two molecules of each of the core histones H3, H2A, H2B and H4 around which 147bp of DNA are wrapped in 1.75 turns of a left handed superhelix. In the nucleosome the globular domains of the four core histones are folded in a compact hydrophobic core, whereas the N-termini extend into solution (Luger et al, 1997) (Figure 1).
KeywordsHistone Modification Polycomb Group Heterochromatin Formation Heterochromatin Domain Cell BioI
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