||S-Adenosylmethionine synthetase (AdoMet synthetase) gene was cloned and characterized from the dinoflagellate Crypthecodinium cohnii. AdoMet synthetase is an enzyme that catalyzes the formation of S-Adenosylmethionine (AdoMet) which, as a main methyl group donor, plays a central role in transmethylation reactions. AdoMet is involved in the biosynthesis of polyamines, and is involved in the biosynthetic pathway for toxins. Moreover, AdoMet-mediated methylation of nucleic acids is known to have regulatory effects on DNA transcription and chromosome structure. Since dinoflagellates are eukaryotic organisms with many prokaryotic cytological features including the lack of histones and nucleosomes, permanently condensed chromosomes, it is interesting to study the role of AdoMet synthetase in dinoflagellates cell cycle progression and AdoMet-mediated DNA methylation on gene expression. During the cell cycle, both the transcript and protein levels of AdoMet synthetase peaked at growth phase and decreased when cells entered S phase. These suggested that AdoMet synthetase has an essential role in growth. An increase in digestibility of methylation-sensitive enzymes on AdoMet synthetase gene was observed following DNA methylation inhibitors L-ethionine and 5-azacytidine treatments, suggesting the presence of cytosine methylation sites within CCGG sequence of the AdoMet synthetase gene. Two hours ahead of the cell cycle delay at the exit of G2/M phase upon inhibition of DNA methylation with 5-azacytidine, a reduction of AdoMet synthetase expression was observed. Another DNA methylation inhibitor, L-ethionine, was able to delay the cell cycle at the entry of S phase with a corresponding reduction of AdoMet synthetase expression. DL-alpha-difluoromethylornithine (DFMO), an inhibitor of polyamine synthetase enzyme, ornithine decarboxylase, caused the reduction of AdoMet synthetase expression. All of the results suggest that AdoMet synthetase is required for the cell cycle progression.