URM1 (ubiquitin related modifier 1) functions as both a sulfur carrier protein and an ubiquitin-like modifier, representing an evolutionary link between prokaryotic sulfur carriers and eukaryotic ubiquitin-like proteins 1. As a sulfur carrier, URM1 is essential for 2-thiolation of mcm(5)S(2)U at wobble positions of cytosolic tRNAs (tRNA-Lys, tRNA-Glu, and tRNA-Gln) through thiocarboxylation at its C-terminus by MOCS3, followed by sulfur transfer to tRNA 2. Additionally, URM1 acts as an ubiquitin-like protein that undergoes covalent conjugation to lysine residues of target proteins including MOCS3, ATPBD3, CTU2, USP15, and CAS through a process called urmylation 1. This dual functionality is mediated by the E1-like enzyme Uba4, which activates URM1 through sequential adenylation, thioesterification, and thiocarboxylation 3. Urmylation is enhanced by oxidative stress and involves coupling UBL-conjugation with persulfidation of cysteine residues in target proteins, highlighting URM1's central role in redox regulation and cellular stress responses 3. The thiocarboxylated form serves as substrate for both tRNA thiolation and protein conjugation, making URM1 unique among ubiquitin-like proteins in its ability to participate in both RNA and protein modification pathways 4.