Breaking the Histone Code of Silence: The Propagation and Blocking of Heterochromatin

Eukaryotic chromosomes consist of blocks of transcriptionally active and inactive regions. Heterochromatic regions of chromatin are generally transcriptionally repressed, and this repressive structure can be propagated along the chromatin fiber. Here I review recent insights on the role of histone modifications in the propagation of repressive chromatin structure, focusing on the creation of heterochromatin protein binding sites by the deacetylation and methylation of nucleosomes, and molecular models detailing the mechanism of propagation. The role of chromatin boundary elements in halting this propagation is discussed in relation to the model systems described. The results suggest that specific combinations of histone modifications constitute a histone code that promotes the propagation of heterochromatin, and by breaking this code along a nucleosomal array, propagation can be stopped to allow expression of downstream genes.