The human genome chromatin can be classified into euchromatin and heterochromatin, which have high and low transcription activities, respectively. In the classical view, it was believed that euchromatin has an open and decondensed structure, while heterochromatin is highly condensed.

A research team led by Professor Kazuhiro Maeshima of Genome Dynamics Laboratory (NIG), including a graduate student (JSPS Research Fellow DC2) Shiori Iida, SOKENDAI graduate student Masa A. Shimazoe, Technical Stuff Sachiko Tamura, and Assistant Professor Satoru Ide have put forward the model that euchromatin essentially forms condensed domains with sizes ranging from 100-300 nm in diameter based on new evidence from genomics and advanced imaging studies. They discuss this novel view of euchromatin in the cell and how the revealed organization is relevant to genome functions.

Physically condensed domains provide a higher-order regulation of transcription, while the extended fiber loops do not. Condensed domains likely hinder the accessibility of large transcription complexes to their target sequence located in the inner core of chromatin domains.

This exclusion effect can repress unintended gene expression. Although such a core of condensed domains seems to be inaccessible, condensed domains have the liquid-like property, allowing small transcription factors to penetrate the core of domains. Such small transcription factors may help a target sequence to float up to the domain surface to be transcribed. The authors suggest that condensed chromatin domains can work as Lego blocks of mitotic chromosomes during cell division, which create a smoother process for chromosome assembly and disassembly.

This work was supported by JSPS Fellowship, JSPD grants (JP21H02453, JP22H05606, JP21H02535, JP20H05936, JP16H06279(PAGS)), JSPS Research Fellow (JP23KJ0996(DC2)), JST SPRING (JPMJSP2104).

Figure: Euchromatic condensed domains provide higher-order regulation of transcription by physically excluding large transcription complexes (green spheres) from the inner core of the domains. Liquid-like property inside a condensed domain gives a certain degree of accessibility for gene expression. Small transcription factors interact with the target gene (open green circle nucleosomes) inside the inner core and cause it to float up to the domain surface to be transcribed. The condensed chromatin domains can work as Lego blocks of mitotic chromosomes during cell division.