Integration and expression of several molecular forms of Rous sarcoma virus DNA used for transfection of mouse cells.

To assess the factors required for integration and expression of retroviral DNA, we have examined viral DNA, RNA, and protein in NIH/3T3 mouse cells transformed by transfection with various forms of cloned Rous sarcoma virus (RSV) DNA. Linear RSV DNA molecules, derived from circular DNA containing two long terminal repeats (LTRs) and permuted by cleavage at the SacI restriction endonuclease site in the leader sequence, were integrated near the ends of the linear molecule, with the LTRs on the 3' side of the src gene. Integration of a subgenomic RSV DNA fragment containing the viral src gene without intact LTRs also occurred near the ends of the linear molecule. Head-to-tail tandem arrays of RSV DNA species were observed in some transformed cell lines that received fully digested DNA and in all cell lines that received DNA ligated to produce oligomers before transfection. Closed circular RSV DNA, with one or two LTRs, integrated without apparent specificity within several regions of the viral genome. After transfection with SacI-permuted RSV DNA still linked to arms of the lambda bacteriophage vector DNA, bacteriophage sequences were joined to host DNA. Transformed cell lines produced by transfection with the various forms of RSV DNA produced similar levels of viral src protein, although the efficiency of successful transformation varied by at least two orders of magnitude. Analyses of viral polyadenylated RNA, together with the patterns of viral DNA in transformed cells, indicated that viral DNA can be integrated and expressed without regard to LTR sequences, with adjacent host DNA presumably supplying signals required for the promotion and processing of functional src mRNA.