P granules and other RNA/protein bodies are membrane-less organelles that may 
assemble by intracellular phase separation, similar to the condensation of water 
vapor into droplets. However, the molecular driving forces and the nature of the 
condensed phases remain poorly understood. Here, we show that the Caenorhabditis 
elegans protein LAF-1, a DDX3 RNA helicase found in P granules, phase separates 
into P granule-like droplets in vitro. We adapt a microrheology technique to 
precisely measure the viscoelasticity of micrometer-sized LAF-1 droplets, 
revealing purely viscous properties highly tunable by salt and RNA 
concentration. RNA decreases viscosity and increases molecular dynamics within 
the droplet. Single molecule FRET assays suggest that this RNA fluidization 
results from highly dynamic RNA-protein interactions that emerge close to the 
droplet phase boundary. We demonstrate than an N-terminal, arginine/glycine 
rich, intrinsically disordered protein (IDP) domain of LAF-1 is necessary and 
sufficient for both phase separation and RNA-protein interactions. In vivo, RNAi 
knockdown of LAF-1 results in the dissolution of P granules in the early embryo, 
with an apparent submicromolar phase boundary comparable to that measured in 
vitro. Together, these findings demonstrate that LAF-1 is important for 
promoting P granule assembly and provide insight into the mechanism by which 
IDP-driven molecular interactions give rise to liquid phase organelles with 
tunable properties.