Histone methyltransferase. Methylates 'Lys-9' of histone H3. H3 'Lys-9' methylation represents a specific tag for epigenetic transcriptional repression. The silencing mechanism via DNA CpNpG methylation requires the targeting of chromomethylase CMT3 to methylated histones, probably through an interaction with an HP1-like adapter. By its function, KYP is directly required for the maintenance of the DNA CpNpG and asymmetric methylation. Involved in the silencing of transposable elements. N(6)-methyl-L-lysyl(9)-[histone H3] + S-adenosyl-L-methionine = H(+) + N(6),N(6)-dimethyl-L-lysyl(9)-[histone H3] + S-adenosyl-L-homocysteine L-lysyl(9)-[histone H3] + S-adenosyl-L-methionine = H(+) + N(6)-methyl-L-lysyl(9)-[histone H3] + S-adenosyl-L-homocysteine Interacts with H3 histone. Associates with centromeric constitutive heterochromatin and at a lower level with regions of euchromatin. Expressed in leaves stems and flowers. Although the SET domain contains the active site of enzymatic activity, both pre-SET and post-SET domains are required for methyltransferase activity. In the pre-SET domain, Cys residues bind 3 zinc ions that are arranged in a triangular cluster; some of these Cys residues contribute to the binding of two zinc ions within the cluster. Mutations in the KYP/SUVH4 gene decrease the level of histone H3-K9 dimethylated, trimethylated or dimethylated in association with H3-K14Ac by factors of 4,3 and 3, respectively. The level of monomethylated H3-K9 is unchanged. Such mutations lead to a drastic decrease of cytosine methylation at CpNpG sites, causing the reactivation of endogenous retrotransposons. The KRYPTONYTE methyltransferase name was given according to its involvement in SUPERMAN gene silencing. Belongs to the class V-like SAM-binding methyltransferase superfamily. Histone-lysine methyltransferase family. Suvar3-9 subfamily.