Heterochromatin and Euchromatin

Abstract

Abstract Eukaryotes are characterised by the extensive packaging of their genomes, initially in a nucleosomal array, and further into higher order domains. Differential packaging is used as a mechanism of gene regulation, with stable silencing of large domains achieved by packaging the deoxyribonucleic acid (DNA) into a heterochromatic structure. Chromosome rearrangements and transgene insertions that misplace euchromatic genes near or within the heterochromatin result in silencing of the euchromatic genes, testifying to a distinct heterochromatin assembly that can antagonise transcription. Most heterochromatic regions are rich in repetitious sequences, frequently derived from transposable elements, and such packaging helps to silence such elements. Domains of heterochromatin and euchromatin are defined by specific covalent modifications of histones and, in some cases, DNA, as well as by associations with a specific subset of nonhistone chromosomal proteins. Chromosomal domains may be targeted for heterochromatin formation by specific noncoding ribonucleic acids (RNAs) . Key Concepts: The chromatin of eukaryotes is differentially packaged into domains of euchromatin and heterochromatin. Displacing a euchromatic gene to near or within the heterochromatin often results in silencing of the euchromatic gene in some of the cells in which it should be active, resulting in a variegating phenotype. Euchromatin and heterochromatin are distinguishable biochemically by different covalent modifications of histones (and in some cases DNA) and by distinct nonhistone proteins. Members of the HP1a chromo domain protein family bind methylated histone H3 and interact with the H3K9 histone methyltransferase to organise transcriptionally repressive heterochromatin. The piRNA pathway is implicated in targeting transposon silencing through local heterochromatin formation.