Figure 1.

The cellular dynamics of Ubiquitin. Four genes encode ub, Uba52 and Uba80, which are ub-ribosome subunit fusion genes (ribosome subunits represented by blue and red ovals), as well as Ubb and Ubc, which are polyubiquitin genes encoding for tandem ubs. These ub precursor proteins are processed to generate free ub, which can be ligated to substrates either in a mono- or poly-ubiquitin arrangement. Ub can be liberated from its substrate through the activity of DUB enzymes. The resulting free ub can circulate between the nucleus and the cytoplasm, and be utilized for several processes. In the nucleus ub is ligated to histone H2B via the activity of RNF20/40 in a FACT, PAF1, RNA polymerase II (Pol II) transcription-dependent manner. The DUB module of SAGA removes ub from histone H2Bub1 in a transcription-independent manner, contributing to the cellular pool of free ub. TSS: transcription start site.