Sequences in the long terminal repeats of the Moloney murine sarcoma virus-124 genome which control transforming gene function

Use of electroporation for high-molecular-weight DNA-mediated gene transfer

Electroporation was used to introduce high-molecular-weight DNA into murine hematopoietic cells and NIH3T3 cells. CCRF-CEM cells were stably transfected with SV2NEO plasmid and the genomic DNA from G-418-resistant clones (greater than 65 kb) was introduced into mouse bone marrow and NIH3T3 cells by electroporation. NEO sequences and expression were detected in the hematopoietic tissues of lethally irradiated mice, with 24% of individual spleen colonies expressing NEO. The frequency of genomic DNA transfer into NIH3T3 cells was 0.25 X 10(-3). Electroporation thus offers a powerful mode of gene transfer not only of cloned genes but also of high-molecular-weight DNA into cells.

Contingent replication assay (CRA) procedure for rapid isolation of enhancers

A rapid procedure for the isolation of functional enhancer sequences consists of the construction of a shotgun DNA library in SV40-based plasmid shuttle vectors which depend on an enhancer for replication, the replication in monkey (CVI) cells of those vectors into which an enhancer sequence was inserted, the selective cleavage of unreplicated vectors by DpnI and the recovery of the replicated vectors by transfection into Escherichia coli. We describe conditions for the fusion of protoplasts to CVI cells, under which conditions the probability of only one type of plasmid entering a cell is increased and thus complementation and rescue of enhancer-less plasmids are decreased. The effectiveness of the procedure is demonstrated by the recovery of enhancers from bovine papillomavirus and Moloney murine sarcoma virus.

Development of an amphotropic, high-titer retrovirus vector expressing the dihydrofolate reductase gene and conferring methotrexate resistance

Altered mouse dihydrofolate reductase gene (DHFRR) was expressed in murine cells using Abelson murine leukemia provirus genome as a prototype vector. A cDNA clone of DHFRR was inserted into a plasmid structure containing retroviral transcriptional as well as packaging signals. The recombinant plasmid was transfected into psi-2 ecotropic cells and the transient virus was used to infect amphotropic PA-12 cells. Recombinant virus (ABL-DHFRR) was detected in the culture medium of transfected PA-12 cells and was free of helper virus. The ABL-DHFRR was capable of conferring methotrexate (MTX) resistance to a variety of cells in culture. The titer of ABL-DHFRR virus was at least tenfold higher than other DHFR retroviruses. The ABL-DHFRR virus titer was increased by selection at increasing concentrations of MTX. The presence of the DHFRR in the virus-infected cells was confirmed by assays which showed reduced inhibition of enzyme activity by MTX. A helper-virus-free, amphotropic, high-titer retrovirus containing the altered DHFR was obtained which may be of use as a dominant selectable marker in infecting hematopoietic progenitor cells.