Dual function of the tRNA(m super(5)U sub(54)) methyltransferase in tRNA maturation

A 5-methyluridine (m super(5)U) residue at position 54 is a conserved feature of bacterial and eukaryotic tRNAs. The methylation of U sub(54) is catalyzed by the tRNA(m super(5)U sub(54)) methyltransferase, which in Saccharomyces cerevisiae is encoded by the nonessential TRM2 gene. In this study, we identified four different strains with mutant forms of tRNA sub(CGA) super(Ser). The absence of the TRM2 gene in these strains decreased the stability of tRNA sub(CGA) super(Ser) and induced lethality. Two alleles of TRM2 encoding catalytically inactive tRNA(m super(5)U sub(54))methyltransferases were able to stabilize tRNA sub(CGA) super(Ser) in one of the mutants, revealing a role for the Trm2 protein per se in tRNA maturation. Other tRNA modification enzymes interacting with tRNA sub(CGA) super(Ser) in the maturation process, such as Pus4p, Trm1p, and Trm3p were essential or important for growth of the tRNA sub(CGA) super(Ser) mutants. Moreover, Lhp1p, a protein binding RNA polymerase III transcripts, was required to stabilize the mutant tRNAs. Based on our results, we suggest that tRNA modification enzymes might have a role in tRNA maturation not necessarily linked to their known catalytic activity.