In S. cerevisiae, the Trm140p methyltransferase is responsible for m3C formation in tRNA-Ser and Thr isoacceptors23,24. Interestingly, the fission yeast S. pombe expresses two Trm140 homologs encoded by the Trm140 and Trm141 genes that are separately responsible for catalyzing m3C in tRNA-Thr and tRNA-Ser, respectively17. In S. cerevisiae, Trm140p recognizes tRNA-Thr substrates via a sequence element encompassing nucleotides 35–37 of the anticodon loop that also includes the t6A modification at position 3725. The recognition of tRNA-Ser isoacceptors by yeast Trm140 homologs is also dependent upon modification of position 37, which can be either t6A or i6A depending on the tRNA-Ser isoacceptor17,25. Notably however, S. cerevisiae Trm140p also requires an interaction with seryl-tRNA synthetase in order to methylate the corresponding seryl-tRNA25. This utilization of the seryl-tRNA synthetase ensures the proper catalysis of m3C on all tRNA serine isotypes since the seryl-tRNA synthetase has evolved to recognize the unusually long variable loop and diverse tertiary structure elements present in the various tRNA-Ser species26,27. Recent studies in Trypanosoma brucei have also uncovered an unusual mechanism by which TRM140 interacts with the ADAT2/3 deaminase
