Biological cells are highly organized, with numerous subcellular compartments. 
Phosphorylation has been hypothesized as a means to control the 
assembly/disassembly of liquid-like RNA- and protein-rich intracellular bodies, 
or liquid organelles, that lack delimiting membranes. Here, we demonstrate that 
charge-mediated phase separation, or complex coacervation, of RNAs with cationic 
peptides can generate simple model liquid organelles capable of reversibly 
compartmentalizing biomolecules. Formation and dissolution of these liquid 
bodies was controlled by changes in peptide phosphorylation state using a 
kinase/phosphatase enzyme pair. The droplet-generating phase transition 
responded to modification of even a single serine residue. Electrostatic 
interactions between the short cationic peptides and the much longer polyanionic 
RNAs drove phase separation. Coacervates were also formed on silica beads, a 
primitive model for localization at specific intracellular sites. This work 
supports phosphoregulation of complex coacervation as a viable mechanism for 
dynamic intracellular compartmentalization in membraneless organelles.