Histone H3 lysine 9 methylation and HP1γ are associated with transcription elongation through mammalian chromatin

Methylation of histones modulates chromatin structure and function. Whereas methylation of histone H3 on lysines 4, 36, and 79 has been linked with gene activation, methylation of H3 on lysines 9 and 27 and histone H4 on lysine 20 is associated with heterochromatin and some repressed genes within euchromatin. Here, we show that H3K9 di- and trimethylation occur in the transcribed region of active genes in mammalian chromatin. This modification is dynamic, as it increases during activation of transcription and is rapidly removed upon gene repression. Heterochromatin Protein 1 gamma (HP1 gamma), a protein containing a chromodomain that recognizes H3K9 methylation, is also present in the transcribed region of all active genes examined. Both the presence of HP1 gamma and H3K9 methylation are dependent upon elongation by RNA polymerase II. These findings demonstrate novel roles for H3K9 methylation and HP1 gamma in transcription activation.