Evidence for DNA-mediated nuclear compartmentalization distinct from phase separation.

Area of Science:

• Molecular Biology
• Virology
• Cell Biology

Background:

• Cellular transcription factors, including RNA Polymerase II (Pol II), form concentrated hubs via multivalent interactions, often involving intrinsically disordered regions.
• During Herpes Simplex Virus (HSV) infection, viral replication compartments (RCs) concentrate host Pol II into membraneless domains, resembling liquid-liquid phase separation.
• The precise mechanism by which these viral RCs organize cellular machinery remains unclear.

Purpose of the Study:

• To elucidate the mechanism of host Pol II enrichment within HSV replication compartments (RCs).
• To investigate the role of protein-DNA interactions in the organization of RCs.
• To understand how viral infection alters nuclear organization and gene activity.

Main Methods:

• Comparative analysis of HSV RCs with known phase-separated condensates.
• Investigation of protein diffusion and DNA binding dynamics within RCs.
• Assessment of viral genome accessibility and nucleosome occupancy.

Main Results:

• HSV RCs utilize a distinct mechanism involving unrestricted, non-specific protein-DNA interactions, outcompeting host chromatin.
• The viral genome is largely nucleosome-free, significantly increasing DNA accessibility for proteins like Pol II.
• Non-specific DNA binding leads to anisotropic protein behavior, causing local factor accumulation despite unrestricted diffusion.

Conclusions:

• Viral replication compartments organize cellular factors through a mechanism driven by non-specific DNA binding, distinct from liquid-liquid phase separation.
• Increased DNA accessibility in RCs profoundly influences the exploration and accumulation of DNA-binding proteins.
• These findings highlight the significant impact of non-specific DNA interactions on gene activity and nuclear organization during viral infection.