Posttranscriptional tRNA modifications are essential for proper gene expression, and defects in the enzymes that perform tRNA modifications are associated with numerous human disorders. Throughout eukaryotes, 2′- O -methylation of residues 32 and 34 of the anticodon loop of tRNA is important for proper translation, and in humans, a lack of these modifications results in non-syndromic X-linked intellectual disability. In yeast, the methyltransferase Trm7 forms a complex with Trm732 to 2′- O -methylate tRNA residue 32 and with Trm734 to 2′- O -methylate tRNA residue 34. Trm732 and Trm734 are required for the methylation activity of Trm7, but the role of these auxiliary proteins is not clear. Additionally, Trm732 and Trm734 homologs are implicated in biological processes not directly related to translation, suggesting that these proteins may have additional cellular functions. To identify critical amino acids in Trm732, we generated variants and tested their ability to function in yeast cells. We identified a conserved RRSAGLP motif in the conserved DUF2428 domain of Trm732 that is required for tRNA modification activity by both yeast Trm732 and its human homolog, THADA. The identification of Trm732 variants that lack tRNA modification activity will help to determine if other biological functions ascribed to Trm732 and THADA are directly due to tRNA modification or to secondary effects due to other functions of these proteins.

Posttranscriptional modification of tRNA is critical for efficient protein translation and proper cell growth, and defects in tRNA modifications are often associated with human disease. Although most of the enzymes required for eukaryotic tRNA modifications are known, many of these enzymes have not been identified and characterized in several model multicellular eukaryotes. Here, we present two related approaches to identify the genes required for tRNA modifications in multicellular organisms using primer extension assays with fluorescent oligonucleotides. To demonstrate the utility of these approaches we first use expression of exogenous genes in yeast to experimentally identify two TRM1 orthologs capable of forming N2,N2-dimethylguanosine (m2,2G) on residue 26 of cytosolic tRNA in the model plant Arabidopsis thaliana. We also show that a predicted catalytic aspartate residue is required for function in each of the proteins. We next use RNA interference in cultured Drosophila melanogaster cells to identify the gene required for m2,2G26 formation on cytosolic tRNA. Additionally, using these approaches we experimentally identify D. melanogaster gene CG10050 as the corresponding ortholog of human DTWD2, which encodes the protein required for formation of 3-amino-3-propylcarboxyuridine (acp3U) on residue 20a of cytosolic tRNA. We further show that A. thaliana gene AT2G41750 can form acp3U20b on an A. thaliana tRNA expressed in yeast cells, and that the aspartate and tryptophan residues in the DXTW motif of this protein are required for modification activity. These results demonstrate that these approaches can be used to study tRNA modification enzymes.

Post‐transcriptional tRNA modifications are required for efficient protein translation. Proteins involved in forming tRNA modifications are being studied to understand their role and function. In yeast, the Trm7 methyltransferase forms a complex with Trm732 to modify tRNA at position 32. In humans, lack of the Trm7 homolog, FTSJ1, has been linked to intellectual disabilities. The human Trm732 homolog, THADA, is associated with type 2 diabetes, PCOS, and cancers. Little is known about the function of Trm732, although we have shown that one conserved motif in Trm732 is important for tRNA modification. We are identifying other residues important for function using two strategies. First, we compare Trm732 proteins of different organisms using protein alignments to determine conserved regions to mutate for testing. Using this strategy, AIN801 was determined to be an important region for the function of Trm732 and we are continuing to look at other regions that may be of importance. Second, we are expressing randomly mutated Trm732 variants in a sick strain that lacks Trm732. Lack of rescue of strain indicates that the mutation present is harmful to protein function. DNA from these colonies is extracted and sequenced to determine the mutation that causes loss of Trm732 function. Understanding the functions of Trm7 and Trm732 in yeast could aid in our understanding of their human homologs, potentially providing insight into the causes of diseases.

Transfer RNAs undergo post‐transcriptional modifications, including many which occur at or near the anticodon loop and are necessary for protein synthesis. In Saccharomyces cerevisiae, Trm7 is required for 2’‐O‐methylation of tRNA and requires interaction with Trm732 and Trm734 for modifications at positions 32 and 34, respectively. These proteins are widely conserved in eukaryotes, and mutations in the human ortholog of TRM7, FTSJ1, can cause non‐syndromic X‐linked intellectual disability by decreasing or eliminating methylation activity. Through a genetic assay, we have found Trm7 residues required for interaction with Trm732 or Trm734 and the subsequent methylation of tRNAPhe. These spot test assays showed that the Trm7 Y138R variant abrogates interaction with Trm734 and the Trm7 I144A variant abrogates interaction with Trm732. To verify that these genetic results are due to impaired interaction of Trm7 variants with its binding partners, we are performing immunoprecipitation assays. In addition, we have begun to study interactions of the individual proteins and complexes with tRNAPhe through biochemical methods such as electrophoretic mobility shift assays, to determine the role of each protein in tRNA binding. Through this study of Trm7 variants, we can better understand the interaction of Trm7 with Trm734 and Trm732, which will facilitate the study of conserved homologs in humans.