Meta-Biol

• Pathways

In the cytoplasm TSR3 transfers an aminocarboxypropyl group from S-adenosylmethionine (AdoMet) to the N(3) position of N(1)-methylpseudouridine at nucleotide 1248 of 18S rRNA yielding N(1)-methyl-N(3)-aminocarboxylpropylpseudouridine-1248 (Meyer et al. 2016). Prior to this reaction, the SNORA13 or ACA13 H/ACA snoRNP (homologue of the snR35 snoRNP in yeast) in the nucleus converts uridine-1248 to pseudouridine-1248 and the EMG1 component of the small subunit processome in the nucleus methylates the N(1) position of pseudouridine-1248. The 18SE precursor of 18S rRNA containing N(1)-methylpseudouridine is then exported to the cytosol as part of the 40S pre-ribosomal subunit. TSR3 is believed to act on the 18SE precursor rather than the mature 18S rRNA because interference with the activity of TSR3 results in an accumulation of 18SE precursor (Meyer et al. 2016).