The present invention relates to a system and method for controlling peptide display valency on virus-like particles (VLPs), especially including MS2 VLPs. In this method, large amounts of wild-type and low quantities of single-chain dimer coat proteins may be produced from a single RNA. Valency is controlled in immunogen (vaccine) production by providing a system that allows the production of large amounts of wild-type and low quantities of single-chain dimer coating proteins from a single RNA, allowing facile adjustment of display valency levels on VLPs, especially MS2 VLPS over a wide range, from few than one- on average- to as many as ninety per particle. This facilitates the production of immunogens and vaccines, including VLPs exhibiting low valency. Nucleic acid constructs useful in the expression of virus-like particles are disclosed, comprised of a coat polypeptide of MS2 modified by insertion of a heterologous peptide, wherein the heterologous peptide is displayed on the virus-like particle and encapsidates MS2 niRNA. Nucleic acid constructs are also disclosed which are useful in the expression of virus-like particles comprised of a coat polypeptide of PP7 modified by insertion of a heterologous peptide, wherein the heterologous peptide is displayed on the virus-like particle and encapsidates PP7 mRNA.La présente invention concerne un système et un procédé de régulation de la valence de présentation de peptides sur des particules de type virus (VLP), incluant spécialement les VLP MS2. Dans ce procédé, de grandes quantités de protéines de type sauvage et de faibles quantités de protéines denveloppe dimériques à simple chaîne peuvent être produites à partir dun ARN unique. La valence est régulée dans la production dun immunogène (vaccin) par la fourniture dun système qui permet la production de grandes quantités de protéines de type sauvage et de faibles quantités de protéines denveloppe dimériques à simple chaîne à partir dun ARN unique permettant lajusteme
