  We propose a description for the quasi-equilibrium self-assembly of small,
single-stranded (ss) RNA viruses whose capsid proteins (CPs) have flexible,
positively charged, disordered tails that associate with the negatively charged
RNA genome molecules. We describe the assembly of such viruses as the interplay
between two coupled phase-transition like events: the formation of the protein
shell (the capsid) by CPs and the condensation of a large ss viral RNA
molecule. Electrostatic repulsion between the CPs competes with attractive
hydrophobic interactions and attractive interaction between neutralized RNA
segments mediated by the tail-groups. An assembly diagram is derived in terms
of the strength of attractive interactions between CPs and between CPs and the
RNA molecules. It is compared with the results of recent studies of viral
assembly. We demonstrate that the conventional theory of self-assembly, which
does describe the assembly of \textit{empty} capsids, is in general not
applicable to the self-assembly of RNA-encapsidating virions.
