  Many single-stranded (ss) RNA viruses self assemble from capsid protein
subunits and the nucleic acid to form an infectious virion. It is believed that
the electrostatic interactions between the negatively charged RNA and the
positively charged viral capsid proteins drive the encapsidation, although
there is growing evidence that the sequence of the viral RNA also plays a role
in packaging. In particular the sequence will determine the possible secondary
structures that the ssRNA will take in solution. In this work, we use a mean
field theory to investigate how the secondary structure of the RNA combined
with electrostatic interactions affects the efficiency of assembly and
stability of the assembled virions. We show that the secondary structure of RNA
may result in negative osmotic pressures while a linear polymer causes positive
osmotic pressures for the same conditions. This may suggest that the branched
structure makes the RNA more effectively packaged and the virion more stable.
