Gene amplification in methotrexate-resistant mouse cells. IV. Different DNA sequences are amplified in different resistant lines

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Abstract
DNA was purified from double minutes isolated from MTX–resistant EL4/8 mouse lymphoma cells, digested to completion with Bam H1 restriction edonuclease and cloned in λ–1059. The properties of the library suggest that the DNA from which it was made was not detectably contaminated with non–dm chromosome material, and that the library is essentially complete for sequences contained in Bam H1 restriction fragments between 9 and 19 kb. The inserts of some selected λ–recombinats were subcloned in pBR328 or pAT153 to separate sequences of differing repetition frequency. Clones representative of different classes of sequences were used as probes to Southern transfers of Bam H1 digested total nuclear DNAs of various MTX–resistant cell lines. The results clearly show that the amplified unit of each cell line has a unique structure, and that different amplified units differ widely in their sequence composition.