Cellular granules lacking boundary membranes harbor RNAs and their associated proteins and play diverse roles controlling the timing and location of protein synthesis. Formation of such granules was emulated by treatment of mouse brain extracts and human cell lysates with a biotinylated isoxazole (b-isox) chemical. Deep sequencing of the associated RNAs revealed an enrichment for mRNAs known to be recruited to neuronal granules used for dendritic transport and localized translation at synapses. Precipitated mRNAs contain extended 3′ UTR sequences and an enrichment in binding sites for known granule-associated proteins. Hydrogels composed of the low complexity (LC) sequence domain of FUS recruited and retained the same mRNAs as were selectively precipitated by the b-isox chemical. Phosphorylation of the LC domain of FUS prevented hydrogel retention, offering a conceptual means of dynamic, signal-dependent control of RNA granule assembly. Display omitted Display omitted ► A crystallized small molecule recruits mRNAs into granule-like aggregates ► The granule-like aggregates contain known granule RNAs ► Aggregates from brain lysates are enriched in mRNAs encoding synaptic proteins ► 3′ UTR lengths of recruited RNAs are significantly longer than average Aggregation of RNA-binding proteins, including FUS, from mammalian cells within hydrogels retains mRNAs previously associated with RNA granules. Phosphorylation of FUS influences mRNA retention suggesting a mechanism for regulating granule assembly and composition.