Histone Chaperone FACT Coordinates Nucleosome Interaction through Multiple Synergistic Binding Events*

Abstract

Background: The histone chaperone FACT binds and reorganizes nucleosomes during critical cellular processes.

Results: FACT binds histones, DNA, and mono- and tri-nucleosomes with high affinity. FACT reduces non-nucleosomal histone/DNA interactions.

Conclusion: Multiple regions of FACT strategically bind target sites on nucleosomes to coordinate (dis)assembly.

Significance: The thermodynamic parameters guiding multiple FACT/nucleosome interaction(s) coincide with reorganization events.

Abstract

In eukaryotic cells, DNA maintenance requires ordered disassembly and re-assembly of chromatin templates. These processes are highly regulated and require extrinsic factors such as chromatin remodelers and histone chaperones. The histone chaperone FACT ( facilitates chromatin transcription) is a large heterodimeric complex with roles in transcription, replication, and repair. FACT promotes and subsequently restricts access to DNA as a result of dynamic nucleosome reorganization. However, until now, there lacked a truly quantitative assessment of the critical contacts mediating FACT function. Here, we demonstrate that FACT binds histones, DNA, and intact nucleosomes at nanomolar concentrations. We also determine roles for the histone tails in free histone and nucleosome binding by FACT. Furthermore, we propose that the conserved acidic C-terminal domain of the FACT subunit Spt16 actively displaces nucleosomal DNA to provide access to the histone octamer. Experiments with tri-nucleosome arrays indicate a possible mode for FACT binding within chromatin. Together, the data reveal that specific FACT subunits synchronize interactions with various target sites on individual nucleosomes to generate a high affinity binding event and promote reorganization.