Degradation of intracellular proteins through the ubiquitin and ATP-dependent proteolysis pathway involves several steps. Initially, ubiquitin is covalently linked to the proteolytic substrate in an ATP-requiring reaction. Proteins marked by ubiquitin may then be selectively lysed in a reaction that also requires ATP (for reviews see refs 1-3). A major question concerns the structural features of a protein that make it a specific substrate for ubiquitin-mediated degradation. It was shown that a free α-NH2 group is one important feature of the protein structure recognized by the ubiquitin ligation system4,5, and that the half-life in vivo of a protein with an exposed amino terminus depends on its amino terminal residue6. We have previously demonstrated that transfer RNA (tRNA) is essential for conjugation of ubiquitin and for the subsequent degradation of proteins with acidic amino termini (aspartate or glutamate)7,8. We now show that tRNA is required for post-translational conjugation of arginine to acidic amino termini of proteins, a modification that is essential for their degradation by the ubiquitin pathway.