Histone-lysine n-methyltransferase ATX1 is encoded by the ATX gene. Methylates 'Lys-4' of histone H3. H3 'Lys-4' methylation represents a specific tag for epigenetic transcriptional activation, which is required to maintain the active state of class A (AP1 and AP2), class B (PI and AP3) and class C (AG) floral homeotic genes at early stages of flower development. It regulates floral organ identity and flowering transition and functions as a receptor for the lipid messenger phosphatidylinositol 5-phosphate (PI5P), which regulates negatively its transcriptional activation activity.