Rna Compaction in the Presence of Polyvalent Cations

Abstract

The effects of polyvalent cations on the effective size and charge of double-stranded DNA (dsDNA) have been well studied. In the presence of polyvalent cations, dsDNA in dilute solution undergoes a single-molecule, first-order phase transition, otherwise called condensation: more explicitly, upon onset of 90% neutralization of the phosphate backbone, the DNA undergoes discontinuous compaction into tightly wound toroids. However, the effects of these cations on long single-stranded RNAs (ssRNA) have not been well characterized. In this study we use centrifugation methods to examine the effective size of long ssRNAs in solutions of increasing concentration of the tetravalent cation spermine. In contrast to the case of dsDNA, we find only a continuous decrease in the size of ssRNA upon increase in spermine concentration. However, the decrease is significant enough to suggest that RNA molecules longer than viral genomes can be packaged in vitro into virus-like vectors for gene delivery.