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|Molar mass||281.27 g·mol−1|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N6-Methyladenosine (m6A ) is an abundant modification in mRNA and is found within some viruses, and most eukaryotes including mammals, insects, plants and yeast. It is also found in tRNA, rRNA, and small nuclear RNA (snRNA) as well as several long non-coding RNA, such as Xist.
Adenosine methylation is directed by a large m6A methyltransferase complex containing METTL3 as the SAM-binding sub-unit. In vitro, this methyltransferase complex preferentially methylates RNA oligonucleotides containing GGACU and a similar preference was identified in vivo in mapped m6A sites in Rous sarcoma virus genomic RNA and in bovine prolactin mRNA.
In budding yeast (Sacharomyces cerevisiae), the homologue of METTL3, IME4 is induced in diploid cells in response to nitrogen and fermentable carbon source starvation and is required for mRNA methylation and the initiation of correct meiosis and sporulation. mRNAs of IME1 and IME2, key early regulators of meiosis, are known to be targets for methylation, as are transcripts of IME4 itself.
Mutations of MTA, the Arabidopsis thaliana homologue of METTL3, results in embryo arrest at the globular stage. A >90% reduction of m6A levels in mature plants leads to dramatically altered growth patterns and floral homeotic abnormalities.
Mapping of m6A in human and mouse RNA has identified over 18,000 m6A sites in the transcripts of more than 7,000 human genes with a consensus sequence of [G/A/U][G>A]m6AC[U>A/C] consistent with the previously identified motif. The localization of individual m6A sites in many mRNAs is highly similar between human and mouse, and transcriptome-wide analysis reveals that m6A is found in regions of high evolutionary conservation. m6A is found within long internal exons and is preferentially enriched within 3’ UTRs and around stop codons. m6A within 3’ UTRs is also associated with the presence of microRNA binding sites; roughly 2/3 of the mRNAs which contain an m6A site within their 3’ UTR also have at least one microRNA binding site. Furthermore, precise m6A mapping by m6A-CLIP/IP (in multiple tissues/cultured cells of mouse and human) revealed that a majority of m6A locates in the last exon of mRNAs, and the m6A enrichment around stop codons is a coincidence that many stop codons locate round the start of last exons where m6A is truly enriched. The major presence of m6A in last exon (>=70%) allows the potential for 3'UTR regulation, including alternative polyadenylation.
m6A is susceptible to dynamic regulation both throughout development and in response to cellular stimuli. Analysis of m6A in mouse brain RNA reveals that m6A levels are low during embryonic development and increase dramatically by adulthood. Additionally, silencing the m6A methyltransferase significantly affects gene expression and alternative RNA splicing patterns, resulting in modulation of the p53 (also known as TP53) signalling pathway and apoptosis.
The importance of m6A methylation for physiological processes was recently demonstrated. Inhibition of m6A methylation via pharmacological inhibition of cellular methylations or more specifically by siRNA-mediated silencing of the m6A methylase Mettl3 led to the elongation of the circadian period. In contrast, overexpression of Mettl3 led to a shorter period. The mammalian circadian clock, composed of a transcription feedback loop tightly regulated to oscillate with a period of about 24 hours, is therefore extremely sensitive to perturbations in m6A-dependent RNA processing, likely due to the presence of m6A sites within clock gene transcripts.
The obesity risk gene, FTO, encodes the first identified m6A demethylase. FTO mutations have been associated with increased risk for obesity and type 2 diabetes, which implicates m6A in important physiological pathways related to human disease. FTO knockdown with siRNA leads to increased amounts of m6A in poly(A) RNA, whereas overexpression of FTO results in decreased amounts of m6A in human cells. FTO partially localizes to nuclear speckles, which supports the notion that m6A in nuclear RNA is a major physiological substrate of FTO. The consequences of FTO-guided demethylation are unknown, but it is likely to affect the processing of pre-mRNA, other nuclear RNAs, or both. The discovery that FTO functions as a cellular m6A demethylase suggests that increased FTO activity in patients with FTO mutations leads to abnormally low levels of m6A in target mRNAs, which through as-yet undefined pathways contributes to the onset of obesity and related diseases.
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