Functions as a protein modifier covalently attached to lysine residues of substrate proteins, as well as a sulfur carrier in molybdenum cofactor (MoCo) biosynthesis. The protein modification process is termed sampylation and involves the formation of an isopeptide bond between the SAMP1 C-terminal glycine carboxylate and the epsilon-amino group of lysine residues on target proteins. May serve as a proteolytic signal in the cell to target proteins for degradation by proteasomes. The C-terminal glycine is likely acyl-adenylated (-COAMP) by UbaA, and also probably thiocarboxylated (-COSH) to function in sulfur transfer. Cells lacking this gene do not grow anaerobically with DMSO and do not show DMSO reductase activity, but their growth in the presence of oxygen and at 50 degrees Celsius is not affected. SAMP2 and SAMP3 cannot complement the samp1 mutation. tRNA thiolation is not affected.