Plasmidal maintenance of composite DNA derived from polyoma related plasmid, L factor

Production of an immunoglobulin gene product by the plasmid expression vector L factor in mouse myeloma cells

L factor, originally discovered in a subclone of mouse L cells, is a multicopy mammalian plasmid whose structure is related to that of polyoma. When a composite DNA consisting of L factor, pBR, bacterial neo, and an immunoglobulin (kappa) gene was introduced into mouse myeloma cells, the DNA was established as plasmids in the cells without rearrangement or integration into the chromosomes. The plasmid-bearing myeloma cells produced kappa mRNA and the gene product, kappa immunoglobulin, which were apparently derived from the gene located on plasmid L factor. These results suggest that L factor can be used as a plasmid expression vector for studies on gene expression and production of biologically active substances in mammalian cells.

Amplification of plasmid (L factor) DNA and increased production of a plasmid gene product (gamma-interferon) in mouse L cells

It has previously been reported that composite DNAs derived from L factor, a polyoma-related mammalian plasmid, can be established in several mouse cell lines after transfection. Here, we report that the copy number of a plasmid composite DNA consisting of L factor, pBR DNA, dihydrofolate reductase (dhfr) gene, and gamma-interferon (gamma-IFN) gene was increased more than 10-fold after two successive adaptations of the plasmid-bearing mouse L cells to increasing concentrations of methotrexate (MTX), an inhibitor of dhfr. The structure of the amplified L factor plasmid remained intact during prolonged cell culture, but the copy number remained to be amplified only when the selective pressure (presence of MTX in the medium) has been exerted during the culture. Cells bearing the amplified plasmid produced a higher level of gamma-IFN compared with the original clone, which was likely to be derived from the plasmid gamma-IFN gene amplified along with L factor and the dhfr gene.

Induction of CAT gene expression on a plasmid vector (L factor) by retinoic acid in mouse embryonal carcinoma (F9) cells

We reported previously that composite DNA constructed from a mammalian plasmid (L factor) and foreign gene can be reestablished as a plasmid in mouse embryonal carcinoma (F9) cells after transfection and the plasmid-bearing F9 cells undergo normal in vitro differentiation in response to retinoic acid, an inducer for F9 cell differentiation. We constructed F9 cells bearing plasmidal L factor DNA in which a reporter (chloramphenicol acetyltransferase; CAT) gene was placed under the control of a differentiation-responsive viral (Moloney murine leukemia virus or simian virus 40) enhancer-promoter. When such plasmid-bearing cells were treated with retinoic acid, the CAT gene was inducibly expressed. These results indicate that mammalian gene expression can be studied with the plasmidal expression vector which is structurally dissociated from complex chromosomes.

Structural and functional analysis of a polyoma-related mammalian plasmid (L factor): the enhancer activity and plasmid establishment

L factor is a unique plasmid DNA which was originally discovered in a subclone (B822) of mouse L cells at a high copy number (more than 5,000 copies/cell). The presence of L factor caused no detectable abnormalities to the plasmid-bearing cells. We determined the total DNA sequence of the L factor I (and a part of L factor II) and compared it with that of polyoma DNA. Both DNA are common to the general construction of DNA frames such as early, late and noncoding regions, suggesting the two to be closely related. On the other hand, the L factor DNA sequences differ substantially from that of polyoma in the DNA sequences corresponding to the polyoma large T antigen, capsid proteins and a portion of the enhancer region. In order to investigate the mechanism of plasmid establishment of L factor, we compared the enhancer activity, capacity of DNA replication and efficiency of plasmid establishment of L factor with those of polyoma. The results indicate that L factor enhancer activity and DNA replication capacity were considerably lower than those of polyoma, suggesting that these altered (lowered) activities associated with L factor contribute to the plasmidal establishment and stable maintenance of L factor.

Establishment of composite DNA derived from L factor as a plasmid in mouse embryonal carcinoma (F9) cells

We have recently reported a mammalian cell plasmid (L factor) whose structure is related to that of polyomavirus (T. Kusano, H. Uehara, H. Saito, K. Segawa, and M. Oishi, Proc. Natl. Acad. Sci. USA 84:1789-1793, 1987). When composite DNA constructed from L factor and a foreign gene was introduced into mouse embryonal carcinoma (F9) cells by transfection, the DNA was reestablished in the cells as a plasmid. The reestablished plasmid DNA in F9 cells could be rescued in Escherichia coli. The plasmid-bearing cells underwent normal in vitro differentiation in response to retinoic acid. The efficiency of plasmid establishment of the L-factor-derived DNA and transcriptional and transient replicational activities were compared with those of similar composite DNA constructed from polyomavirus and an embryonal carcinoma mutant of polyomavirus which is permissive in F9 cells. The results suggest an inverse relationship between the efficiency of the plasmid establishment and the activity of gene expression controlled by the intrinsic enhancer-promoter of the DNA.