Post-translational modification of histone tails is thought to modulate 
higher-order chromatin structure. Combinations of modifications including 
acetylation, phosphorylation and methylation have been proposed to provide marks 
recognized by specific proteins. This is exemplified, in both mammalian cells 
and fission yeast, by transcriptionally silent constitutive pericentric 
heterochromatin. Such heterochromatin contains histones that are generally 
hypoacetylated and methylated by Suv39h methyltransferases at lysine 9 of 
histone H3 (H3-K9). Each of these modification states has been implicated in the 
maintenance of HP1 protein-binding at pericentric heterochromatin, in 
transcriptional silencing and in centromere function. In particular, H3-K9 
methylation is thought to provide a marking system for the establishment and 
maintenance of stably repressed regions and heterochromatin subdomains. To 
address the question of how these two types of modifications, as well as other 
unidentified parameters, function to maintain pericentric heterochromatin, we 
used a combination of histone deacetylase inhibitors, RNAse treatments and an 
antibody raised against methylated branched H3-K9 peptides. Our results show 
that both H3-K9 acetylation and methylation can occur on independent sets of H3 
molecules in pericentric heterochromatin. In addition, we identify an RNA- and 
histone modification-dependent structure that brings methylated H3-K9 tails 
together in a specific configuration required for the accumulation of HP1 
proteins in these domains.