Spreading of Chromatin Modifications

Reader enzymes act in collaboration with writer or eraser enzymes to create chromatin modifications and spread the chromatin changes for a substantial distance along a chromosome. 

The process, which functions similarly with writer and eraser enzymes, is described here using the reader and writer combination as an example.  

The process begins when a transcriptional regulatory protein binds to a specific DNA sequence. After binding, the regulatory protein recruits a writer enzyme to that specific site on a chromosome. 

The writer enzyme then starts to add marks to the core histones. 

Once marks have been added to one or more neighboring nucleosomes, a multiprotein complex containing a ‘reader’ enzyme that recognizes these marks, binds tightly to the newly modified nucleosome.

The binding activates the ‘writer’ enzyme of the multiprotein complex and positions it near an adjacent nucleosome, enabling it to add a new mark.

Working in concert, the reader and writer enzymes of the complex catalyze a series of many ‘read and write’ cycles. These cycles spread a chromatin modification, such as chromatin condensation, along a chromosome.

The boundary between adjacent chromatin domains like euchromatin and heterochromatin, which have different structures and functions, is marked by specific DNA sequences called ‘barrier sequences.’ 

These barrier sequences act as the binding sites for various barrier proteins, which block the propagation of heterochromatin into euchromatin by mediating different barrier actions. 

For example, some barrier proteins recruit histone-modifying enzymes which erase the histone marks that are required for the spreading of the heterochromatin.

Some barrier proteins tightly bind to the group of nucleosomes, covering them up and thereby making the covered euchromatin resistant to heterochromatin spreading.

Yet another barrier protein tethers a region of chromatin to large fixed sites, such as nuclear pores, and forms a barrier that stops the spread of heterochromatin.