H2A mono-ubiquitination differentiates functions of FACT in nucleosome assembly and disassembly

The histone chaperone FACT plays an essential role in transcription and DNA replication by its dual functions on nucleosome assembly to maintain chromatin integrity and nucleosome disassembly to destabilize nucleosome and facilitate its accessibility simultaneously. UbH2A has been suggested to repress transcription by preventing the recruitment of FACT at early elongation process. However, up to date, how ubH2A directly affects FACT on nucleosome assembly and disassembly remains elusive. In this study, we demonstrated that the dual functions of FACT are differently regulated by ubH2A. The H2A ubiquitination does not affect function of FACT in nucleosome assembly and FACT can deposit ubH2A H2B dimer on tetrasome to form intact nucleosome. However, ubH2A greatly restricts FACT binding on nucleosome and inhibits its activity of nucleosome disassembly. Interestingly, deubiquitination of ubH2A rescues the nucleosome disassembly function of FACT to activate gene transcription. Our findings provide mechanistic insights of how H2A ubiquitination affects FACT in breaking nucleosome and maintaining its integrity, which sheds light on the biological function of ubH2A and various activity of FACT under different chromatin states.