METHIONINE SYNTHASE 1 Is Involved in Chromatin Silencing by Maintaining DNA and Histone Methylation

DNA methylation and histone modification are important epigenetic marks that co-regulate gene expression and genome stability. To identify factors involved in chromatin silencing, we carried out a forward genetic screen for mutants that release the silenced Pro35S:LUCIFERASE (35SP-LUC) in Arabidopsis (Arabidopsis thaliana). We identified an epigenetic regulator, METHIONINE SYNTHASE 1 (ATMS1), which catalyzes the synthesis of methionine in the one-carbon metabolism pathway. The ATMS1 mutation releases the silenced 35SP-LUC and the majority of endogenous genes and transposons. The effect of ATMS1 on chromatin silencing is related to decreased levels of DNA methylation (CG, CHG, and CHH) and H3K9me2. The ATMS1 mutation caused a significant decrease in the ratio of S-adenosylmethionine (SAM)/S-adenosylhomocysteine (SAH). Exogenous application of methionine rescued the phenotype of atms1-1. ATMS1 plays a predominant role in DNA and histone methylations among the three methionine synthetase homologs. These results suggest that ATMS1 is required for DNA and histone methylations through its function in the one-carbon metabolism pathway, indicating the complex interplay between metabolism and epigenetic regulation.