Ubiquitin related modifier 1 (Urm1) is a ubiquitin related protein that modifies proteins in the yeast ubiquitin-like urmylation pathway []. Structural comparisons and phylogenetic analysis of the ubiquitin superfamily has indicated that Urm1 has the most conserved structural and sequence features of the common ancestor of the entire superfamily []. Urm1 acts as a sulfur carrier required for 2-thiolation of mcm5S2U at tRNA wobble positions of cytosolic tRNA(Lys), tRNA(Glu) and tRNA(Gln) [].
This entry includes adenylyltransferase and sulfurtransferase MOCS3 and its homologue, Uba4, from yeasts.In budding yeasts, Uba4 receives sulphur from Nfs1 and Tum1 []. Uba4 then forms an acyl adenylate intermediate with the C-terminal of the Urm1 protein, and then forms an acyl disulfide bond between Uba4 Cys397 and the Urm1 C-terminal glycin []. Subsequently, the activated Urm1 acts as a sulphur carrier in the process of eukaryotic transfer RNA (tRNA) thiolation []. MOCS3 plays a central role in 2-thiolation of mcm5S2U at tRNA wobble positions of cytosolic tRNA(Lys), tRNA(Glu) and tRNA(Gln) []. It is also essential during biosynthesis of the molybdenum cofactor [, ]. MOCS3 acts by mediating the C-terminal thiocarboxylation of sulfur carriers Urm1 and MOCS2A (the small subunit of molybdopterin synthase) [, , , ].
