The exotic thymidine modification 5-hydroxymethyluridine in dinoflagellate Amphidinium carterae

Dinoflagellate chromosomes are extraordinary, as their organization is independent of architectural nucleosomes unlike typical eukaryotes and shows a cholesteric liquid crystal state. 5-hydroxymethyluridine (5hmU) is present at unusually high levels and its function remains an enigma in dinoflagellates chromosomal DNA. Here, we demonstrate that 5hmU exhibits content variations in different dinoflagellates and is generated at the poly-nucleotide level through hydroxylation of thymidine. Importantly, we identified the enzyme, which is a putative dinoflagellate TET/JBP homologue, catalyzing 5hmU production using either in vivo or in vitro biochemical assay. Based on the near-chromosomal level genome assembly of dinoflagellate Amphidinium carterae, we depicted a comprehensive 5hmU landscape and found that most 5hmU peaks share a conserved TG-rich motif, and are significantly enriched in repeat elements, which mark partially overlapping regions with 5-methylcytosine (5mC) sites. Moreover, inhibition of 5hmU via dioxygenase inhibitor leads to transcriptional activation of 5hmU-marked transposable elements (TEs), implying that 5hmU appears to serve as epigenetic marks for silencing retrotransposon. Together, our results revealed the biogenesis, genome-wide landscape and molecular function of dinoflagellate 5hmU, providing mechanic insight into the function of this enigmatic DNA mark. ### Competing Interest Statement The authors have declared no competing interest.