The nucleocapsid domain is responsible for the ability of spleen necrosis virus (SNV) gag polyprotein to package both SNV and murine leukemia virus RNA

Murine leukemia virus (MLV)-based vector RNA can be packaged and propagated by the proteins of spleen necrosis virus (SNV). We recently demonstrated t hat MLV proteins cannot support the replication of an SNV-based vector; RNA analysis revealed that MLV proteins cannot efficiently package SNV-based v ector RNA. The domain in Gag responsible for the specificity of RNA packagi ng was identified using chimeric gag-pol expression constructs. A competiti ve packaging system was established by generating a cell line that expresse s one viral vector RNA containing the MLV packaging signal (Psi) and anothe r viral vector RNA containing the SNV packaging signal (E), The chimeric ga g-pol expression constructs were introduced into the cells, and vector tite rs as well as the efficiency of RNA packaging were examined. Our data confi rm that Gag is solely responsible for the selection of viral RNAs. Furtherm ore, the nucleocapsid (NC) domain in the SNV Gag is responsible for its abi lity to interact with both SNV E and MLV Psi Replacement of the SNV NC with the MLV NC generated a chimeric Gag that could not package SNV RNA but ret ained its ability to package MLV RNA. A construct expressing SNV gag-MLV po l supported the replication of both MLV and SNV vectors, indicating that th e gag and pol gene products from two different viruses can functionally coo perate to perform one cycle of retroviral replication Viral titer data indi cated that SNV cia-acting elements are not ideal substrates for MLV pol gen e products since infectious viruses were generated at a lower efficiency. T hese results indicate that the nonreciprocal recognition between SNV and ML V extends beyond the Gag-RNA interaction and also includes interactions bet ween Pol and other cia-acting elements.