RING E3 ligases bind to E2ubiquitin thioester intermediates and catalyse the transfer of ubiquitin to substrates. In addition to binding to the E2 catalytic site, free ubiquitin has been shown to bind to the backside of E2 ligases, which promotes processive polyubiquitin chain formation. Huang and colleagues now show that ubiquitin bound to the backside of the E2 protein UBCH5B enhances RING-dependent ubiquitin transfer. Using a structural and biochemical approach, the authors showed that backside-bound ubiquitin stabilizes a catalytically favourable conformation of the E3–E2ubiquitin complex, which promotes processive polyubiquitin chain formation as well as initial RING-mediated ubiquitin transfer. On the basis of their findings, the authors propose that backside-bound ubiquitin functions as an allosteric activator that enhances RING E3-dependent ubiquitin transfer.