Enzymatic Synthesis and Fractionation of Fluorescent PolyU RNAs

The physical properties of viral-length polyuridine (PolyU) RNAs, which cannot base-pair and form secondary structures, are compared with those of normal-composition RNAs, composed of comparable numbers of each of A, U, G and C nucleobases. In this protocol, we describe how to synthesize fluorescent polyU RNAs using the enzyme polynucleotide phosphorylase (PNPase) from Uridine diphosphate (UDP) monomers and how to fractionate the polydisperse synthesis mixture using gel electrophoresis, and, after electroelution, how to quantify the amount of polyU recovered with UV-Vis spectrophotometry. Dynamic light scattering was used to determine the hydrodynamic radii of normal-composition RNAs as compared to polyU. It showed that long polyU RNAs behave like linear polymers for which the radii scale with chain length as N1/2, as opposed to normal-composition RNAs that act as compact, branched RNAs for which the radii scale as N1/3.