Location and function of retroviral and SV40 sequences that enhance biochemical transformation after microinjection of DNA.

Biochemical transformation of thymidine-kinase-deficient (TK-) mouse L cells is enhanced 20 to 40 fold when microinjected plasmid DNA contains regions of the genomes of Rous sarcoma virus or simian virus 40 in addition to the complete herpes simplex virus tk gene, irrespective of the orientation and location of the enhancer. The enhancing sequence of RSV DNA has been localized to 143 bp that include 88 bp at the 5' end of the long terminal repeat; the enhancing segment of SV40 DNA lies 42-276 bp upstream from the initiation site for early transcription. The RSV enhancer does not provide a favored integration site within microinjected plasmids and does not affect the frequency of integration into host cell DNA. When microinjected cells were grown into cell lines in nonselective medium, few cells containing integrated HSV tk without RSV DNA could grow in selective medium. In contrast, over 80% of cells from lines containing the HSV tk gene linked to the RSV enhancer could grow in selective medium. Thus the RSV enhancer affects the expression rather than the integration of the tk gene.