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The 3-methylcytidine (m 3 C) modification is widely found in eukaryotic species of tRNA Ser , tRNA Thr , and tRNA Arg ; at residue 32 in the anti-codon loop; and at residue e2 in the variable stem of tRNA Ser . Little is known about the function of this modification or about the specificity of the corresponding methyltransferase, since the gene has not been identified. We have used a primer extension assay to screen a battery of methyltransferase candidate knockout strains in the yeast  Saccharomyces cerevisiae , and find that tRNA Thr(IGU)  from  abp140 -Δ strains lacks m 3 C. Curiously, Abp140p is composed of a poorly conserved N-terminal ORF fused by a programed +1 frameshift in budding yeasts to a C-terminal ORF containing an  S -adenosylmethionine (SAM) domain that is highly conserved among eukaryotes. We show that  ABP140  is required for m 3 C modification of substrate tRNAs, since primer extension is similarly affected for all tRNA species expected to have m 3 C and since quantitative analysis shows explicitly that tRNA Thr(IGU)  from an  abp140 -Δ strain lacks m 3 C. We also show that Abp140p (now named Trm140p) purified after expression in yeast or  Escherichia coli  has m 3 C methyltransferase activity, which is specific for tRNA Thr(IGU)  and not tRNA Phe  and occurs specifically at C 32 . We suggest that the C-terminal ORF of Trm140p is necessary and sufficient for activity in vivo and in vitro, based on analysis of constructs deleted for most or all of the N-terminal ORF. We also suggest that m 3 C has a role in translation, since  trm140-Δ trm1-Δ  strains (also lacking m 2,2 G 26 ) are sensitive to low concentrations of cycloheximide.

A domain of the actin binding protein Abp140 is the yeast methyltransferase responsible for 3-methylcytidine modification in the tRNA anti-codon loop