Active genes are tri-methylated at K4 of histone H3

Lysine methylation of histones in vivo occurs in three states: mono-, di- and tri-methyl(1). Histone H3 has been found to be dimethylated at lysine 4 (K4) in active euchromatic regions but not in silent heterochromatic sites(2). Here we show that the Saccharomyces cerevisiae Set1 protein can catalyse di- and tri- methylation of K4 and stimulate the activity of many genes. Using antibodies that discriminate between the di- and tri- methylated state of K4 we show that di- methylation occurs at both inactive and active euchromatic genes, whereas tri- methylation is present exclusively at active genes. It is therefore the presence of a trimethylated K4 that defines an active state of gene expression. These findings establish the concept of methyl status as a determinant for gene activity and thus extend considerably the complexity of histone modifications.