Helpers for efficient encapsidation of SV40 pseudovirions

Plasmid DNA that carries the simian virus 40 (SV40) ori can be packaged as SV40 pseudovirions. The pseudovirions are very efficient in gene transmission into a variety of cell types, including human hemopoietic cells. They are routinely prepared with wild-type (wt) SV40 as a helper. In the present study, several parameters required for the helper function were investigated. Plasmids that carry pBR322 sequences in addition to the late genes of S V40 were inefficient in providing helper functions, presumably because the prokaryotic sequences interfered with expression of the SV40 late genes. Efficient helpers were plasmid pSVPiC [Villarreal and Soo, Mol. Appl. Genet. 3 (1985) 62-71] and an SV40 defective virus SLT3 (presently constructed). Plasmid pSVPiC carries a duplication of the SV40 ori and enhancer regions, and πAN7 sequences. Because of its large size it was not packaged into virion particles. However, it underwent extensive recombination generating infective SV40 particles. Almost no prokaryotic sequences are included in SLT3, that carries the SV40 late gene. In spite of its small size (3.5 kb) it was packaged efficiently, creating defective (T-antigen-negative) SV40 virions. The availability of T-antigen positive and negative pseudovirion mixtures enabled us to suggest that T-antigen drives gene amplification in the target human hemopoietic cells.