Urm1 at the crossroad of modifications

The ubiquitin‐like protein Urm1 can be covalently conjugated to other proteins, such as the yeast thioredoxin peroxidase protein Ahp1p, through a mechanism involving the ubiquitin E1‐like enzyme Uba4. Recent findings have revealed a second function of Urm1 as a sulphur carrier in the thiolation of eukaryotic cytoplasmic transfer RNAs (tRNAs). Interestingly, this new role of Urm1 is similar to the sulphur‐carrier activity of its prokaryotic counterparts, strengthening the hypothesis that Urm1 is a molecular fossil of the ubiquitin‐like protein family. Here, we discuss the function of Urm1 in light of its dual role in protein and RNA modification.