Identification and characterization of the de novo methyltransferases for eukaryotic N6-methyladenine (6mA)

DNA N6-adenine methylation (6mA) is rediscovered as an important epigenetic mark in eukaryotes. We previously identified the MT-A70 family member AMT1 as the maintenance methyltransferase (MTase), but the identity of the de novo MTase(s) for 6mA establishment remains unidentified. The unicellular model eukaryote Tetrahymena thermophila offers a unique opportunity for studying de novo methylation: during sexual reproduction, the 6mA-free zygotic nucleus develops into 6mA-positive new macronucleus (MAC) marked by genome-wide de novo methylation. Here we identified that TET1 and TET2 are essential for de novo 6mA in the new MAC, but not for maintenance 6mA in the somatic MAC. Knockout of TET1 and/or TET2 not only abolished methylation on most methylatable ApT positions, but also attenuated methylation on the rest. The substantial 6mA decrease in turn altered gene expression pattern and reduced the survival rate of sexual progenies, highlighting the importance of dedicated de novo MTase activity.