Recently, a Legionella pneumophila effector protein was shown to have an unprecedented ATP-independent ubiquitin ligase activity that couples phosphoribosylated ubiquitin (PR-Ub) to serine residues of host proteins. A new study published in Cell Research by Qiu et al. reveals that another Legionella effector protein, SidJ, catalyzes deubiquitination of PR-Ub by cleavage of the substrate-linked phosphodiester bond.