The biogenesis of 2-thiouridine derivatives at the wobble position 34 of bacterial tRNA requires a sulfur-relay system that involves multiple sulfur mediators. Cysteine is the donor of sulphur group. The type of proteins that compose this relay systems depends on the organism or cell considered. As far as E. coli MnmA is concerned, at least 6 proteins compose this complex relay system (IscS, TusA, TusB, TusC, TusD and TusE). U34 thiolation appears not to depend on prior posttransciptional modification of C5 atom of the uridine ring and thus can occur any time during the biogenesis of X5s2U34. A crystal structure of the MnmA-tRNA complex has been solved at 3.1 angstroms of resolution.