Amplification of a cloned Chinese hamster dihydrofolate reductase gene after transfer into a dihydrofolate reductase-deficient cell line.

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Abstract
We have transformed a dihydrofolate reductase (DHFR)-deficient Chinese hamster ovary cell line to the DHFR+ phenotype with a recombinant cosmid (cH1) containing a functional Chinese hamster DHFR gene (J.D. Milbrandt et al., Mol. Cell. Biol. 3:1266-1273, 1983). After exposure of cells to successive increases in methotrexate, we have isolated a resistant cell line (JSH-1) that grows in 1 microM methotrexate. We show here that JSH-1 contains 300 to 500 copies of the integrated cosmid and that these copies are located predominantly at one position on a chromosome identified as Z5a. Hybridization analysis of restriction digests of genomic DNA indicates that the cosmid has been integrated intact into the genome and that upon amplification, the original cosmid/genomic junction fragments are also amplified in JSH-1. Furthermore, the pattern of amplified bands observed in ethidium bromide-stained gels indicates that the unit amplified sequence (amplicon) may be as large as 120 to 135 kilobases and therefore includes considerable amounts of flanking DNA in addition to the 45 kilobases of integrated cosmid. We also show that the protein overproduced by the amplified cosmid in JSH-1 comigrates with the 21,000-dalton polypeptide characteristic of the methotrexate-resistant cell line (CHOC 400) from which cH1 was cloned. However, the DHFR mRNA species overproduced in JSH-1 appear to be larger than those detected in CHOC 400, indicating that not all of the normal transcription and processing signals are preserved in the integrated recombinant cosmid.