Heterochromatin revisited

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Abstract
Heterochromatin is a DNA-packaging state that is commonly associated with transcriptional silencing and repression of recombination. Methylation of histone H3 at lysine 9 (H3K9me) and recruitment of heterochromatin protein HP1 are essential steps in heterochromatin assembly. These factors, when targeted to specific sites on chromosomes, can spread, allowing large heterochromatic domains to form. Heterochromatin formation can be initiated by proteins that bind to DNA 'silencer' sequences. It can also be triggered by repetitive DNA elements through the action of the RNAi pathway. Heterochromatin factors serve as a platform to recruit and facilitate the spreading of effector proteins, allowing them to exert regional control. Heterochromatin mediates the spreading of RNAi machinery from nucleation sites to surrounding sequences. Chromosome-bound RNAi 'factories' might selectively produce small interfering RNAs (siRNAs), which are involved in genomic silencing and defense against parasitic elements. Recruitment of histone deacetylases (HDACs) enforces the transcriptional-silencing role of heterochromatin. HDACs seem to have a crucial role in chromatin condensation. Despite the well-described roles of heterochromatin in chromatin compaction and silencing, in some instances, effectors that are recruited by heterochromatin might also facilitate transcription. The heterochromatin platform is dynamic and can be readily altered in response to developmental signals. Boundary elements that limit the spread of heterochromatin have been described in some systems. They seem to work by diverse mechanisms.