Dealing with an Unconventional Genetic Code in Mitochondria: The Biogenesis and Pathogenic Defects of the 5‐Formylcytosine Modification in Mitochondrial tRNA$^{Met}$

Human mitochondria contain their own genome, which uses an unconventional genetic code. In addition to the standard AUG methionine codon, the single mitochondrial tRNA Methionine (mt‐tRNA$^{Met}$) also recognises AUA during translation initiation and elongation. Post‐transcriptional modifications of tRNAs are important for structure, stability, correct folding and aminoacylation as well as decoding. The unique 5‐formylcytosine (f$^{5}$C) modification of position 34 in mt‐tRNA$^{Met}$ has been long postulated to be crucial for decoding of unconventional methionine codons and efficient mitochondrial translation. However, the enzymes responsible for the formation of mitochondrial f$^{5}$C have been identified only recently. The first step of the f$^{5}$C pathway consists of methylation of cytosine by NSUN3. This is followed by further oxidation by ABH1. Here, we review the role of f$^{5}$C, the latest breakthroughs in our understanding of the biogenesis of this unique mitochondrial tRNA modification and its involvement in human disease.