Three ARABIDOPSIS HOMOLOG of TRITHORAX (ATX3, ATX4 and ATX5) Function as H3K4 methyltransferase and Are Redundantly Crucial for Both Sporophyte and Gametophyte Development. Arabidopsis thaliana

The SET domain-containing genes of the Trithorax family encode chromatin-remodeling factors that activate the expression of targeted genes by antagonistically functioning against the Polycomb group. Of 47 Arabidopsis (Arabidopsis thaliana) SDG protein, 31 are known to have histone lysine methytranferase (HKMT) activity, and can be divided into five classes (I to V). Class III consists of five genes that encode homologues of Trithorax, and are therefore named as Arabidopsis Trithorax-like protein 1-5 (ATX1-5). Although ATX1 and ATX2 have been previously identified to be involved in several processes of plant development by mediating H3K4 trimethylation and dimethylation respectively, biological functions of ATX3, ATX4 and ATX5remain uncharacterized. In this study, we show that these three genes are closely correlated with identical protein domain architecture and overlapping expression pattern, and redundantly critical for proper sporophyte and gametophyte development. The concurrent disruption of ATX3, ATX4 and ATX5 causes markedly reductions in total levels of cellular H3K4me2/3, and specific decreases of H3K4me2/3 in thousands of genomic regionsat a genome-wide scale identified by ChIP-seq analysis. In addition, RNA-seq analysis shows that transcriptions of thousands of genes are affected in atx3/4/5 mutant, but not strictly correlated with changes in H3K4me2/3 levels. Moreover, we find that H3K4me3 regionsaffected byATX3, ATX4 and ATX5 are largely distinct from SDG2-dependent regionsand exhibit discrete characters. Taken together, these results suggest that ATX3, ATX4 and ATX5functioning separately from SDG2, encode another main enzyme responsible for genome-wide H3K4me2/3 establishment in Arabidopsis and redundantly crucial for regulating gene expression and plant development.