NONRECIPROCAL PSEUDOTYPING - MURINE LEUKEMIA-VIRUS PROTEINS CANNOT EFFICIENTLY PACKAGE SPLEEN NECROSIS VIRUS-BASED VECTOR RNA

It has been documented that spleen necrosis virus (SNV) can package mu rine leukemia virus (MLV) RNA efficiently and propagate MLV vectors to the same titers as it propagates SNV-based vectors. Although the SNV packaging signal (E) and MLV packaging signal (Psi) have little sequen ce homology, similar double-hairpin RNA structures were predicted and supported by experimental evidence. To test whether SNV RNA can be pac kaged by MLV proteins, we modified an SNV vector to be expressed in an MLV-based murine helper cell line. Surprisingly, we found that MLV pr oteins could not support the replication of SNV vectors. The decrease in titer was approximately 2,000- to 20,000-fold in one round of retro viral replication. RNA analysis revealed that SNV RNA was not efficien tly packaged by MLV proteins. RNA hybridization of the cellular and vi ral RNAs indicated that SNV RNA was packaged at least 25-fold less eff iciently than MLV RNA, which was the sensitivity limit of the hybridiz ation assay. The contrast between the MLV and SNV packaging specificit y is striking. SNV proteins can recognize both SNV E and MLV Psi, but MLV can recognize only MLV Psi. This is the first demonstration of two retroviruses with nonreciprocal packaging specificities.