Methylation of position-specific lysine residues in histone N termini is a 
central modification for regulating epigenetic transitions in chromatin. Each 
methylatable lysine residue can exist in a mono-, di-, or trimethylated state, 
thereby extending the indexing potential of this particular modification. Here, 
we examine all possible methylation states for histone H3 lysine 9 (H3-K9) and 
lysine 27 (H3-K27) in mammalian chromatin. Using highly specific antibodies 
together with quantitative mass spectrometry, we demonstrate that pericentric 
heterochromatin is selectively enriched for H3-K27 monomethylation and H3-K9 
trimethylation. This heterochromatic methylation profile is dependent on the 
Suv39h histone methyltransferases (HMTases) but independent of the euchromatic 
G9a HMTase. In Suv39h double null cells, pericentric heterochromatin is 
converted to alternative methylation imprints and accumulates H3-K27 
trimethylation and H3-K9 monomethylation. Our data underscore the selective 
presence of distinct histone lysine methylation states in partitioning 
chromosomal subdomains but also reveal a surprising plasticity in propagating 
methylation patterns in eukaryotic chromatin.