Amith, W. D.; Chen, V. T.; Dutagaci, B.

RNA Polymerase II (Pol II) C-terminal domain (CTD) is known to have crucial roles in regulating transcription. CTD has also been highly recognized for undergoing phase separation, which is further associated with its regulatory functions. However, the molecular interactions that the CTD forms to induce clustering to drive phase separations and how the phosphorylation of CTD affects clustering are not entirely known. In this work, we studied the concentrated solutions of two heptapeptide repeats (2CTDs) models at different phosphorylation patterns, protein, and ion concentrations using all-atom molecular dynamics simulations to investigate clustering behavior and molecular interactions driving the cluster formation. Our results show that salt concentration and phosphorylation patterns play an important role in determining the clustering pattern, specifically at low protein concentrations. The balance between inter- and intra-peptide interactions and counterion coordination together impact the clustering behavior upon phosphorylation.