  We examine the limits on viral composition that are set by the electrostatic
interactions effected by the charge on the viral proteins, the single-stranded
viral RNA molecule and monovalent salt ions in the solution. Within the
mean-field model of viral energetics we demonstrate the prime importance of the
salt concentration for the assembly of a virus. We find that the encapsidation
of the viral RNA molecule is thermodynamically suppressed in solutions with
high concentrations of monovalent salt. This effect is significantly less
important in viruses with proteins whose charge distribution protrudes into the
interior of the capsid, leading to an increase in the stability of such viruses
in solutions with high salt concentrations. The delocalization of positive
charge on the capsid protein arms thus profoundly increases reliability of
viral assembly in high-salt solutions.
