Comparative studies of the expression of linked Escherichia coli gpt gene and BPV-1 DNAs in transfected cells

Abstract

A series of hybrid plasmids, containing two selective markers that can be expressed in mammalian cells have been constructed. These plasmids are derived from the pSV2 gpt recombinant plasmid described by Mulligan and Berg (1980) and contain the entire BPV-1 DNA, or the HindIII- BamHI large transforming fragment (T69) or the early transforming region of polyoma virus DNA. DNA transfers into Fisher rat 3T3 cells were performed either by the calcium phosphate coprecipitation technique, or by protoplast fusion. For all plasmids, the frequency of formation of phenotypically transformed foci or of the expression of the gpt + marker in selective medium have been scored comparatively. Both series of recombinant plasmids gave similar yield of gpt + colonies, whereas BPV plasmids (both entire or T69 subgenome) transformed morphologically rat cells 8–50 times less frequently than their polyoma homologues. Although the pSV2 gpt BPV-1 plasmids can replicate autonomously to high copy number in monkey COS cells, the rate of transcription of the BPV-1 genome in these cells is 10 2 to 10 3 times lower than that of gpt transcribed from the SV40 early promoter. This low rate of transcription may explain the low frequency of transformation by this viral DNA.