The 2′-O-methyltransferase responsible for modification of yeast tRNA at position 4

Abstract

The methylation of the ribose 2′-OH of RNA occurs widely in nature and in all stable RNAs and occurs at five positions in yeast tRNA. 2′-O-methylation of tRNA at position 4 is interesting because it occurs in the acceptor stem (which is normally undermodified), it is the only 2′-O-methylation that occurs in the middle of a duplex region in tRNA, the modification is conserved in eukaryotes, and the features of the tRNA necessary for substrate recognition are poorly defined. We show here that Saccharomyces cerevisiae ORF YOL125w (TRM13) is necessary and sufficient for 2′-O-methylation at position 4 of yeast tRNA. Biochemical analysis of the S. cerevisiae proteome shows that Trm13 copurifies with 2′-O-methylation activity, using tRNA^Gly(GCC) as a substrate, and extracts made from a trm13-Δ strain have undetectable levels of this activity. Trm13 is necessary for activity in vivo because tRNAs isolated from a trm13-Δ strain lack the corresponding 2′-O-methylated residue for each of the three known tRNAs with this modification. Trm13 is sufficient for 2′-O-methylation at position 4 in vitro since yeast Trm13 protein purified after expression in Escherichia coli has the same activity as that produced in yeast. Trm13 protein binds substrates tRNA^His and tRNA^Gly(GCC) with K_D values of 85 ± 8 and 100 ± 14 nM, respectively, and has a K_M for tRNA^His of 10 nM, but binds nonsubstrate tRNAs very poorly (K_D > 1 μM). Trm13 is conserved in eukaryotes, but there is no sequence similarity between Trm13 and other known methyltransferases.

Keywords

• tRNA methyltransferase
• Trm13
• 2′-O-methylation
• S. cerevisiae
• tRNA processing
• tRNA modification