Acts as a sulfur carrier required for 2-thiolation of mcm(5)S(2)U at tRNA wobble positions of cytosolic tRNA(Lys), tRNA(Glu) and tRNA(Gln) (By similarity). Serves as sulfur donor in tRNA 2-thiolation reaction by being thiocarboxylated (-COSH) at its C-terminus by MOCS3 (By similarity). The sulfur is then transferred to tRNA to form 2-thiolation of mcm(5)S(2)U (By similarity). Also acts as a ubiquitin-like protein (UBL) that is covalently conjugated via an isopeptide bond to lysine residues of target proteins such as Jafrac1, Ciao1, Eip71CD and GILT1 (PubMed:26715182, PubMed:28953965). The thiocarboxylated form serves as substrate for conjugation and oxidative stress specifically induces the formation of UBL-protein conjugates (By similarity). Plays a role in the regulation of JNK signaling in response to oxidative stress (PubMed:26715182). tRNA modification; 5-methoxycarbonylmethyl-2-thiouridine-tRNA biosynthesis. Interacts with cer. Ubiquitously expressed during all embryonic stages, third instar larval imaginal disk, early pupal stages and in the adult (at protein level). C-terminal thiocarboxylation occurs in 2 steps, it is first acyl-adenylated (-COAMP) via the hesA/moeB/thiF part of the MOCS3 homolog, then thiocarboxylated (-COSH) via the rhodanese domain of the MOCS3 homolog. Lethal; most flies die by the pupal stages (PubMed:26715182). Escapers are small, less active, display a significantly reduced fecundity, shortened lifespan, increased resistance to oxidative stress, up-regulation of the JNK signaling pathway and increased expression of Jafrac1 and GstD1 (PubMed:26715182). Simultaneous knockout of Jafrac1 restores normal sensitivity to oxidative stress (PubMed:26715182). RNAi-mediated knockdown leads to higher tolerance to oxidative stress (PubMed:26715182). Belongs to the URM1 family. Extended N-terminus.