Structure and function of a conserved tRNA modifying complex

The structure and function of RNA is regulated by post-transcriptional modifications which have therefore far reaching implications for RNA functionality. RNA modifying enzymes are consequently crucial for cellular physiology. Aberrant activity of RNA modifying enzymes is directly associated with a variety of human pathologies, ranging from developmental disorders to cancer and mitochondriopathies. Despite their importance and connection to human disease, our understanding of how RNA modifying enzymes function and the basis for their substrate selectivity is still at its infancy. Although all classes of RNAs are post transcriptionally modified, tRNAs are considered to be the most densely modified. Anti codon loop modifications play a role in influencing the core function of tRNAs in translation and are therefore central for tRNA activity. Here we provide a comprehensive mechanistic analysis of a highly conserved tRNA-modifying enzyme complex that mediates the formation of an essential and universal modification at the anti codon loop of its tRNA substrate. We uncovered the structural basis for how tRNA is engaged by the complex and how the tRNA modification site is guided into the active site of the modifying enzyme. We thus provide a novel model for the structure-function of an evolutionarily conserved RNA modifying enzymatic complex.