de novo methylation of N6-methyladenine (6mA) showcased in the unicellular eukaryote Tetrahymena Epigenetic Chromatin Regulation

DNA N⁶-adenine methylation (6mA) is rediscovered as an important epigenetic mark in eukaryotes. It however remains unclear what the de novo methyltransferases (MTases) are and how they fulfill their biological functions. The binucleated Tetrahymena thermophila happens to possess a unique time window for the study of de novo methylation, when the 6mA-free zygotic nucleus develops into 6mA-positive new macronucleus (MAC). In this study, we found that the MT-A70 family members, AMT2 and AMT5, are not required for the maintenance 6mA in the vegetative MAC but for de novo 6mA in the new MAC. As a consequence loss of AMT2 and/or AMT5 reduces the fitness of sexual progeny.