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. Author manuscript; available in PMC: 2013 May 6.
Published in final edited form as: Pharmacogenomics J. 2012 Jan 17;12(5):361–367. doi: 10.1038/tpj.2011.64

Figure 1.

Figure 1

Euchromatin and Heterochromatin. (a) Acetylated lysine 9 of histone 3 (H3K9acetyl) results in an open configuration of chromatin otherwise known as ‘euchromatin’. H3K9me2 is synthesized by histone methytransferases (HMT such as G9a, GLP, SETDB1) and initiates the assembly of a restrictive chromatin also termed ‘heterochromatin’. H3K9me2 in turn serves as a high affinity binding site for the attachment of the platform protein HP1.45 HP1 collapses the nucleosome assembly and further recruits other transcriptionally restrictive proteins to this site.46 The spatial spread of heterochromatin is a result of HP1 dimerization between nucleosomes.47 (b) The spread of heterochromatin can be contiguous as in (c) or can assemble non-contiguously (in trans, orange vs. blue colored strands) and relocate to a region of the nucleus that is restrictive to transcription. Facultative heterochromatin is an intermediate stage of restriction that retains the ‘faculty’ for promoter activity. Constitutive heterochromatin is the extreme restrictive state along this continuum; beyond a point, HDAC inhibition may be therapeutically ineffective.