RNA demethylase that mediates oxidative demethylation of different RNA species, such as mRNAs, tRNAs and snRNAs, and acts as a regulator of fat mass, adipogenesis and energy homeostasis. Specifically demethylates N(6)-methyladenosine (m6A) RNA, the most prevalent internal modification of messenger RNA (mRNA) in higher eukaryotes. M6A demethylation by FTO affects mRNA expression and stability. Also able to demethylate m6A in U6 small nuclear RNA (snRNA). Mediates demethylation of N(6),2'-O-dimethyladenosine cap (m6A(m)), by demethylating the N(6)-methyladenosine at the second transcribed position of mRNAs and U6 snRNA. Demethylation of m6A(m) in the 5'-cap by FTO affects mRNA stability by promoting susceptibility to decapping. Also acts as a tRNA demethylase by removing N(1)-methyladenine from various tRNAs. Has no activity towards 1-methylguanine. Has no detectable activity towards double-stranded DNA. Also able to repair alkylated DNA and RNA by oxidative demethylation: demethylates single-stranded RNA containing 3-methyluracil, single-stranded DNA containing 3-methylthymine and has low demethylase activity towards single-stranded DNA containing 1-methyladenine or 3-methylcytosine. Ability to repair alkylated DNA and RNA is however unsure in vivo. Involved in the regulation of fat mass, adipogenesis and body weight, thereby contributing to the regulation of body size and body fat accumulation. Involved in the regulation of thermogenesis and the control of adipocyte differentiation into brown or white fat cells (By similarity). Regulates activity of the dopaminergic midbrain circuitry via its ability to demethylate m6A in mRNAs (By similarity). 2-oxoglutarate + a 5'-end (N(7)-methyl 5'-triphosphoguanosine)-(N(6),2'-O-dimethyladenosine) in mRNA + O2 = a 5'-end (N(7)-methyl 5'-triphosphoguanosine)-(2'-O-methyladenosine) in mRNA + CO2 + formaldehyde + succinate 2-oxoglutarate + an N(6)-methyladenosine in mRNA + O2 = an adenosine in mRNA + CO2 + formaldehyde + succinate 2-oxoglutarate + N(6)-methyladenosine in U6 snRNA + O2 = adenosine in U6 snRNA + CO2 + formaldehyde + succinate 2-oxoglutarate + a 5'-end (N(7)-methyl 5'-triphosphoguanosine)-(N(6),2'-O-dimethyladenosine) in U6 snRNA + O2 = a 5'-end (N(7)-methyl 5'-triphosphoguanosine)-(2'-O-methyladenosine) in U6 snRNA + CO2 + formaldehyde + succinate 2-oxoglutarate + an N(1)-methyladenosine in tRNA + O2 = an adenosine in tRNA + CO2 + formaldehyde + succinate Binds 1 Fe(2+) ion per subunit. Activated by ascorbate. Inhibited by N-oxalylglycine, fumarate and succinate. Monomer. May also exist as homodimer. Localizes mainly in the nucleus, where it is able to demethylate N(6)-methyladenosine (m6A) and N(6),2'-O-dimethyladenosine cap (m6A(m)) in U6 small nuclear RNA (snRNA), N(1)-methyladenine from tRNAs and internal m6A in mRNAs. In the cytoplasm, mediates demethylation of m6A and m6A(m) in mRNAs and N(1)-methyladenine from tRNAs. Ubiquitous (PubMed:17143547, PubMed:18218688). Highly expressed in teeth and weakly in bone (PubMed:17143547). Up-regulated in the hypothalamus after 48 hours fasting. The 3D-structure of the Fe2OG dioxygenase domain is similar to that of the Fe2OG dioxygenase domain found in the bacterial DNA repair dioxygenase alkB and its mammalian orthologs, but sequence similarity is very low. As a consequence, the domain is not detected by protein signature databases. Belongs to the fto family.