The Levels of a Universally Conserved tRNA Modification Regulate Cell Growth

N⁶-Threonylcarbamoyl-adenosine (t⁶A) is a universal modification occurring at position 37 in nearly all tRNAs that decode A-starting codons, including the eukaryotic initiator tRNA (tRNA_i^Met). Yeast lacking central components of the t⁶A synthesis machinery, such as Tcs3p (Kae1p) or Tcs5p (Bud32p), show slow-growth phenotypes. In the present work, we show that loss of the Drosophila tcs3 homolog also leads to a severe reduction in size and demonstrate, for the first time in a non-microbe, that Tcs3 is required for t⁶A synthesis. In Drosophila and in mammals, tRNA_i^Met is a limiting factor for cell and animal growth. We report that the t⁶A-modified form of tRNA_i^Met is the actual limiting factor. We show that changing the proportion of t⁶A-modified tRNA_i^Met, by expression of an un-modifiable tRNA_i^Met or changing the levels of Tcs3, regulate target of rapamycin (TOR) kinase activity and influences cell and animal growth in vivo. These findings reveal an unprecedented relationship between the translation machinery and TOR, where translation efficiency, limited by the availability of t⁶A-modified tRNA, determines growth potential in eukaryotic cells.