Crosstalk between the tRNA methyltransferase Trm1 and RNA chaperone La influences eukaryotic tRNA maturation

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tRNAs undergo an extensive maturation process involving posttranscriptional modifications often associated with tRNA structural stability and promoting the native fold. Impaired posttranscriptional modification has been linked to human disease, likely through defects in translation, mitochondrial function, and increased susceptibility to degradation by various tRNA decay pathways. More recently, evidence has emerged that bacterial tRNA modification enzymes can act as tRNA chaperones to guide tRNA folding in a manner independent from catalytic activity. Here, we provide evidence that the fission yeast tRNA methyltransferase Trm1, which dimethylates nuclear- and mitochondrial-encoded tRNAs at G26, can also promote tRNA functionality in the absence of catalysis. We show that WT and catalytic-dead Trm1 are active in an invivo tRNA-mediated suppression assay and possess RNA strand annealing and dissociation activity invitro, similar to previously characterized RNA chaperones. Trm1 and the RNA chaperone La have previously been proposed to function synergistically in promoting tRNA maturation, yet we surprisingly demonstrate that La binding to nascent pre-tRNAs decreases Trm1 tRNA dimethylation invivo and invitro. Collectively, these results support the hypothesis for tRNA modification enzymes that combine catalytic and noncatalytic activities to promote tRNA maturation, as well as expand our understanding of how La function can influence tRNA modification.