The highly abundant mRNA m1A modification: a new layer of gene regulation in dinoflagellates

The N1-methyladenosine (m 1 A) is a positively charged RNA modification known to disrupt base pairing and influence RNA stability. Despite its limited presence in the mRNA of various organism models, including yeast, mouse, and human, the exact processes of m 1 A biosynthesis, distribution, regulation, and function remain controversial. Dinoflagellates are a major group of single-celled eukaryotic phytoplankton having peculiar crystalline chromosomes. Their genes are arranged in unidirectional gene clusters along the chromosomes and only have minimal transcriptional regulation, implying the involvement of other critical regulatory mechanisms in gene expression. Here, we found that m 1 A rather than m 6 A is the most prevalent mRNA modification in dinoflagellates and asymmetrically distributed along mature transcripts. Utilizing the dinoflagellate species Amphidinium carterae as a study model, we identified 13481 m 1 A peaks characterized by a non-tRNA T-loop-like sequence motif within the transcripts of 10794 genes, many of which are involved in carbon and nitrogen metabolism. With enrichment around stop codon region and 3’ UTR, dinoflagellate mRNA m 1 A exhibits negative correlation with translation efficiency. Notably, nitrogen depletion (N-depletion) treatment led to significant global decrease of mRNA m 1 A amount, causing dramatic variation in translation rates with minimal changes in transcription. Additionally, our analysis uncovered distinctive methylation patterns of m 1 A modification that appears to post-transcriptionally modulate gene expression through regulating translation efficiency. Thus, our findings provide the first comprehensive m 1 A map of dinoflagellate mRNA, shedding light on its crucial role as a post-transcriptional regulatory layer to compensate the degeneration of transcriptional regulation in dinoflagellate. This study also sets the stage for further investigation into the biogenesis and functional significance of mRNA m 1 A in eukaryotes.