Plants Contain a High Number of Proteins Showing Sequence Similarity to the Animal SUV39H Family of Histone Methyltransferases

The SET domain, first identified within and named after proteins encoded by three Drosophila genes [Su(var)3-9, E(z), and Trithorax], is recognized as a signature motif for histone methyltransferases that are involved in epigenetic processes. The SUV39H family of SET domain proteins methylate specifically the residue lysine 9 of histone H3, creating a code for gene silencing. This family of proteins contain at their C termini a unique catalytic domain consisting of pre-SET, SET, and post-SET domains. Sequence homology-based searches identified 15 Arabidopsis, 14 maize, and 12 rice proteins that can be assigned to the SUV39H family. These high numbers in plants are in marked contrast to the situation in animals, in which each species appears to contain only two to three proteins of this family. Our phylogenetic analyses revealed that plant proteins can be classified into seven orthology groups. Representative members of each group can be found in single plant species, suggesting that different group members are evolutionarily conserved to perform specific functions.