Binds tRNA and catalyzes the iron and alpha-ketoglutarate dependent hydroxylation of 5-methylcarboxymethyl uridine at the wobble position of the anticodon loop in tRNA via its dioxygenase domain, giving rise to 5-(S)-methoxycarbonylhydroxymethyluridine. Binds 1 Fe(2+) ion per subunit. Arabidopsis ALKB8 isoform 2 has an extension of 83 amino acids in its N-terminus, but this extension shares no detectable sequence homology with other plant orthologs. The 159 nucleotide DNA sequence encoding the N-terminal part (53 amino acids) of the 83 amino acid extension is highly similar (94% sequence identity) to the Ac-type transposon Tag2. Belongs to the alkB family. Arabidopsis ALKB8 isoform 2 has an extension of 83 amino acids in its N-terminus, but this extension shares no detectable sequence homology with other plant orthologs. The 159 nucleotide DNA sequence encoding the N-terminal part (53 amino acids) of the 83 amino acid extension is highly similar (94% sequence identity) to the Ac-type transposon Tag2.